1) Concerning Super Mario Bros. 3, it's interesting how the rating of the boss fights differ between All the tropes and TV Tropes... (this has probably to do more with the personal experience of two different editors rather than with differences between the websites) While Lemmy is described as difficult here, he is listed as a breather boss in Tv Tropes, while Ludwig is rated as difficult.

My personal experience with the game is again a bit different: I find Roy and Ludwig quite tough as they will often stun-lock me unless Mario/Luigi has a flying power-up. Wendy, too, is annoying, but not quite as hard, as her rings fly at least rather slowly. Morton is probably the easiest, as he traps himself repeatedly in a small pit within the arena, slowing himself down.

In general, the SMB3 Koopaling fights are not as easy as they look, as they immediately attack you as you enter their arenas and you generally need to respond quickly to their moves. (In the New SMB series, they appear to have been slowed down, making them easier.) Since my reaction speed is rather slow (compared to e.g. what you can see on some Youtube videos), I will sometimes get shot or stomped even by Larry or Morton, however Wendy and Lemmy leave a bit more time for me to react because their projectiles are slower. So Wendy/Lemmy aren't that much harder than Larry/Morton/Iggy in my personal experience, as their projectiles are pinballing but still somehow predictable. Roy is nigh-impossible to beat with small Mario, but becomes a cakewalk with a Super Leave or Tanooki Suit, once I manage to keep it through his airship (which is quite hard, too, because the airship itself doesn't supply you with these power-ups). Similarly for Ludwig, although he jumps higher than Roy, making hovering more risky (and staying on the ground slightly less deadly, as his earthquake frequency is a bit lower due to his higher jumps).

In conclusion, there seem to be differences in personal experience of different players which make it hard to say which of the SMB3 Koopalings is the hardest. I think that Wendy, Roy and Ludwig were supposed to be hard, while Morton, Iggy and Lemmy were supposed to be easier than their direct predecessors.

2) Concerning another Mario game, New Super Mario Bros DS, I ask myself if Monty Tank (the boss of world 6) classifies as a That One Boss, or rather a Goddamned Boss. Sure, he is not freaking hard, and you can even cheese him with a mega mushroom (making him a de-facto zero-effort boss). But in his last stage, he becomes a bit annoying, as you have to springboard on a bullet bill to reach him. He then likes to shoot two or three bullet bills directly above each other, making springboarding impossible without getting hurt. So if you're small Mario, you have to wait for the tactical suicide move of shooting a single bullet bill at the right height. The problem is, if you have started his castle from the beginning, you're quite likely to run out of time here. This is much less likely to happen on a successive attempt where you start from the mid-level checkpoint. What kind of boss is this? Does it qualify as That One Boss? The predecessor Petey Piranha of World 5 is significantly easier to beat, for example.

There seems to be a general tendency for Super Mario bosses to be, say, hard when you let them attack before attacking back, but easy when you don't wait for them to do the first move. There is often a way to defeat even the most feary bosses nearly without effort, if you know how.