No matter what, the Moon Lord always gives me the most trouble overall though.
Moon Lord feels more like a slog than anything. I can defeat him usually, but it's just how stinking long it takes to whittle his massive HP down, and that's after doing the pillars to spawn him (yes yes I know pop item etc etc etc, but I need the shards the pillars drop to make the actual equipment AND you need the shards to make the pop item.... so the pop item ain't that useful anyways).
It means dumping 30+ minutes into fighting a boss, once. It's a chore and there's a reason why I've only gotten full moon lord armor -once-. It's so boring.
In my Opinion, I would imagine the very reason they introduced the enraged mechanic for Duke Fishron, was because Pre-1.3. Players could simply create long bridges to run across and dodge all of Fishron's Large Sharknados and overall attacks. With being restricted in the ocean to fight normally, it forces the player to deal with the large vortexes. And can still be managed well with an Arena with several platforms at different heights.
We're talking about Terraria.
You know, that game where using the Terrain (including Terrain you create yourself) in creative ways? Last I checked, this was Terraria, that very game. But wait, the devs go "oh wait, we don't want you "exploiting" terrain by doing the obvious" ... *sigh* what's next, Eye of Cthulhu insta-kills you if you don't fight him while standing on the ground?
The problem with confining players to the Ocean is you really DON'T have very much room to move around (especially in a Small world!). The sharknadoes are freaking huge, they throw the stupid sharks everywhere, he's charging at you constantly... and it just feels like a cheap boss now that you can't dodge him. It used to be that building a bridge was a reasonable way to fight him, because well... obviously. Just like building platforms in the sky for EoC and Skeletron, or the Twins, or Prime...
We're
supposed to be using terrain to help us kill bosses. That's what this whole game is about!
It seems so silly to go "NOPE! Not with THIS boss!" like that. It defies what this game was built for.