Awesome Bosses/Bioware Obsidian

The makers of some of the most popular Western RPGs of the past decade and a half certainly know a thing or two about awesome bosses.

Baldurs Gate series

 * Final battle of Baldurs Gate I.
 * Baldurs Gate II: Shadows of Amn has so many of them. How about...
 * Jierdan Firkraag?
 * "KNOW THAT YOU FACE DEMOGORGON! PRINCE. OF. DEMONS!"
 * Kangaxx. Immune to all weapons below + 4 and all spells, regenerates really fast and has an unlimited supply of Imprisonment.
 * Ol' Jon Irenicus himself. Technically, there is little about Irenicus that distinguishes him from a really tough foe. But, oh, does this show the importance of plot, pacing, and structure. You encounter Irenicus in just enough different ways, in just enough different versions, and his influence in the plot kicks just enough different dogs in different ways. Each encounter seems the last. Then, there build up to the final throwdown is nothing short of amazing. After all of that, beating him for good? It's soooooo satisfying.
 * And, of course, the really nasty Twisted Rune fight. It's almost mandatory when you're soloing a Sorcerer, too.
 * Baldurs Gate II: Throne of Bhaal picks up the awesome boss trend and runs with it...
 * The fight against Sendai. She's tricky, tough, and great deal of fun.
 * The Improved Yaga Shura mod. Thought he was a pushover before? Think again. And he brings his army with him. His entire army.
 * The final battle with the Ascension mod installed. True epic.
 * Ascension converts the final level from a relatively simple marathon fight against Mellissan into a Boss Rush of epic proportions. The first indication the player has of this is when instead of ending her A God Am I speech at the start by attacking the party, Mellissan dismisses you as beneath her notice then
 * Additional mods that are Ascension-compatible allow for things like recruiting Irenicus (think about it, you can have both games' Big Bads at your side in this battle, if you're the good type even having redeemed both), temporarily resurrecting several plot important characters from the past to fight at your side (including Gorion and Yoshimo), oh, and Demogorgon also shows up again. Bad luck for him that he just stepped into the sphere of Bhaal, where you can destroy him for good. This all combines to make the final battle into an ultimate showdown of ridiculously epic degree.

Planescape: Torment

 * Planescape: Torment has, as its approximate halfway point, Ravel Puzzlewell. The environs the fight takes place in are suitably creepy, and the preceding conversation has established her as nothing less than an outright Manipulative Bastard. She also summons giant, corrupted tree-things to do her bidding, and her spells are nothing to sneeze at either.
 * There's also the NEXT big boss you face after
 * The final boss,

Neverwinter Nights series

 * Neverwinter Nights 2 may not be a great game, but the Final Boss and the penultimate boss fight are pretty awesome. First,  Many speeches and lectures are given, and affirmations of loyalty are made. Then you beat up The Dragon and whatever allies he has. Then the real Big Bad comes through the portal, and he  . Then you beat him up. In both cases, you have your entire party fighting--some six to ten level 20 characters.
 * Mask of the Betrayer has the boss fight at the end of chapter 1, against Okku and his Spirit Army. It's quite difficult, even with a party of four. It's recommended that you take out the lesser spirits first.
 * The last battle is a . He attacks the spirits of your past within you, and you have to stop him and chase him down to the very core of your being. Defeat him there, and it's up to you whether Love Redeems, some sacrifices have to be made or you've had enough and you want some revenge.

Star Wars: Knights of the Old Republic series

 * Darth Malak, anyone? The most powerful enemy in the game, alone, clashing with you in a lightsaber duel while aboard his fortress in a manner not unlike Luke vs. Darth Vader in Return of the Jedi. The emotional impact behind this battle - - combined with the fact that it's a friggin' lightsaber duel has to make this one of the most awesome fights in video games.
 * Fighting him for Sith Supremacy was more awesome, insofar you can finish the game with him admitting that you are better, after which you deliver the "And so the apprentice has learned his final lesson" line.
 * Facing off against some of the Sith teachers on Korriban. Not quite as epic, but almost as fun. Depending on how big a bastard you want to be, Jorak Uln can lead to fun times with lightning bolts for one of your douchebag fellow students. Or the final encounter with Yuthura and Uthar -- depending on whether or not you went for the straightforward option, the double-cross, or the double double-cross, it can range from a truly kickass Dual Boss fight, a Melee a Trois, a two-on-one fight with optional backstabby goodness, or an honest-to-God heartwarming Heel Face Turn. Ah, choices, choices...
 * The temple-top fight against Bastila is a jaw dropper, too. Massive Player Punch for a light sider male.
 * Darth Traya in the second Knights of the Old Republic. You think you've won, and you would have, except for the three floating lightsabers Traya summons to kill you.
 * And then there's her dialogue. By TV Tropes definition she may be a Nietzsche Wannabe, but if Nietzsche were a Jedi, she'd have convinced him to curl up and die a long time ago.

Jade Empire series

 * Considering that he was using you as an Unwitting Pawn the entire game? Considering how callously he talks about his lost family, especially when you've got Dawn Star right next to you? It is very satisfying to pound Sun Li into the ground. If you're fond of little Wild Flower, it is also terribly satisfying to kick Ya Zhen to the curb for terrifying the sweet little girl.

Mass Effect series
"Shepard:"
 * The final boss of Mass Effect 2:  . You fight . On foot. Even if it's just , that's still one of the most awesome things in the entire game, and it's fought to some of the most awesome music in the game. Killing the   pretty much confirms that yes, Shepard is the only one who can save the galaxy.   Reduced somewhat if you hit it with the Cain, a boss-killing mushroom-clouding explosive launcher that halves the boss' health if you hit it. (Which is more like taking off 1/4 of its health on the hardest difficulty.) Then again, using the Cain means you kill a giant mecha-Chtulhu with a nuclear bomb in the face. Think about it.
 * Though it's more of a mini-boss, taking down a  on foot when the mission only requires you to survive it for 5 minutes is a Crowning Moment of Awesome, especially on Insanity difficulty. Even the krogan think so!
 * It's awesome in terms of lore, but the actual fight is probably easier than the one that comes after it, especially considering that the above is a three-on-one fight.
 * The  during Tali's recruitment mission. Three different ways to attack the damn thing, with geth closing in on you constantly from every angle, and Tali's theme music blasting in the background.
 * in Lair of the Shadow Broker, at least for Vanguards. can use the same Biotic Charge ability Vanguards have, so much of the battle involves zooming around the battlefield like crazy. Even if you aren't a Vanguard, it's still an awesome fight. Also counts as That One Boss.
 * The fight against
 * In Mass Effect 3, the on Rannoch. Again, it's not actually that hard, but you're facing down . It's amazingly epic and a fitting conclusion to the Rannoch arc.
 * The one on Tuchanka is also pretty epic. The is blocking the way to the Shroud facility, and the chance to destroy it conventionally has been lost. Instead,
 * Kai Leng. After dealing with this slippery bastard throughout the game, it's satisfying to finally lay into him with all your might. The Boss Banter is also entertaining, mostly consisting of Shepard mocking him for his repeated failures, reducing Leng to sputtering fury. Then, after you beat him, you get one of Shepard's best Crowning Moments of Awesome.

Dragon Age series

 * Dragon Age: Origins has a few:
 * The Ogre in the Tower of Ishal. It's not just a "Wake-Up Call" Boss, it's an awesome "Wake-Up Call" Boss. Especially if you're playing a melee character and you get the final blow against it. You leap at it in Bullet Time and stab it to death, tackling it to the ground.
 * The Fade Dream in the Circle Tower is a Scrappy Level to many, but it does have the final boss against the Sloth Demon. It almost seems like the Smug Snake Demon is getting more and more desperate as he changes forms in a vain attempt to defeat you.
 * The battle against . Fighting a powerful and decidedly not Squishy Wizard   who casts some of the most powerful spells in the game and summons Ents into battle? While epic, it's the aftermath of the battle that's really awesome. The boss music is icing on the cake.
 * . Unlike the other  who can be taken down fairly easily   the High Dragon will have you on the edge of your seat. One wrong move and you're dead. This battle rivals the   battles in the Baldur's Gate and Icewind Dale games for their sheer awesomeness.
 * The Archdemon isn't as challenging in terms of gameplay as one might hope, but the scene is epic.
 * Dragon Age II has a few. The Harvester, the High Dragon, and all have moments. The Duel Boss with the  is cool, if more difficult than the group fight.
 * The DLC boss Corypheus is an incredibly epic battle, given that he's

Alpha Protocol

 * While you're still in Saudi Arabia, there's the interception of Al-Samad's missile convoy. You have to take out Shaheed's second-in-command to keep him from alerting the convoy to your presence, so if you sneak up on him, you perform a flying leap to his back, hurling him to his death under the treads of Shaheed's tank, block the convoy's progress across a bridge with a single pistol shot, then have to fight a tank on foot while dodging its missiles. Chances are good you've overlooked the missile launchers so thoughtfully prepared for you.
 * In the main game there are climactic battles at the end of each chapter. Moscow provides the best in and of itself as you close in on Konstatine Brayko, American 80s obsessed Russian mobster who just got done torturing one of your allies. When you get to him, he's on a disco stage and the two of you engage in Pre Ass Kicking One Liners set to a rousing guitar riff. When the battle starts, you realize it's Autograph's Turn Up the Radio (they originally planned to use The Final Countdown). As soon as you manage to drop to drop his health by a quarter, he shoots up on cocaine and comes at you with a knife. Accepted wisdom for this segment is to RUN until his high passes.
 * MAAAAAAAAAARBUUUUUUUUUUUUUURG! is going to be your first thought as the Rome mission's boss fight starts up. After this guy Kicks The Dog in whatever way you choose to let him, busting a cap in his face is going to be satisfying to even the most Ghandi-like Actual Pacifist on a no-kill run. Too bad that even if you put a few hundred assault rifle bullets through him, he gets away...
 * In Taipei you've been hounded by the ruthless and capable Omen Deng since you started trying to stop the assassination of Ronald Sung. Your final confrontation takes place on a construction scaffold, climbing up to balcony facing the platform where Sung is speaking where Deng decides to make his final stand against you.

Fallout: New Vegas

 * Fallout: New Vegas has very few traditional bosses, but the Giant Roboscorpion from Old World Blues is suitably epic. It's the size of a semi truck, has multiple attack modes, and -- importantly -- is fought in an arena that gives you multiple options on how to defeat it, depending on your character's strengths. Also, its onboard nuclear reactor goes critical after it's defeated, giving you a nice mushroom cloud finale.

Dungeon Siege III

 * Dungeon Siege III is full of these. From the fight against Rajani  who sets most of the arena on fire, through the Warbeast battle - a humongous monster in a mine, with enemies pouring in and friendly cannonballs exploding everywhere, the Dapper Old Gent, all the way to a 5 stage brawl against Jayne, the penultimate stage being a 2vs3, where the bosses combo your ass into oblivion! Hands down the best Obsidian boss fights.