Super Mario Bros. 2/YMMV

Demonic Spiders: Phantos, full stop. Nigh-unkillable, can fly through walls, follow you into and out of doors, and is just plain scary looking. In a game series renowned for avoiding Nightmare Fuel, this enemy takes that title and runs with it.
Game Breaker: Luigi's jumping prowess makes many stretches of the game much easier.
- Peach's floating made it even easier.
  - In general; Luigi breaks vertical jumping, Peach breaks horizontal jumping. (Mario's vertical jump height is actually better than Peach's.)
- The Game Boy Advance remake made shells rebound from walls instead of disappearing upon contact, and it now always generates hearts for every enemy it kills, making it ripe for exploit potential. Not to mention throwing the giant Shy Guys and Ninjis for hearts...
Hate Dumb: This game has a small hatedom that showed up merely because of how different it is than other Mario games. At the time, we didn't know - and even on its own merits, it's still pretty well designed. There are some people who like it because it's different than the other Mario games that feature more linear levels. (While not really "non-linear", they aren't "go to the end of the screen" like in the first Mario game.)
- Not to mention, The Irate Gamer review of this is pretty much a good example of Hate Dumb - even if he does bring up one or two valid points such as how utterly chaotic the game gets and the fact that it's not easy (Unless you have a save feature that enables you to practice on later levels)
Hilarious in Hindsight: The POW Block and Starman cameos in DDP could be considered this.
Memetic Mutation: The overworld music. IT BEGINS.