Mine is ocram because he is the biggest challenge. I have console version. I am so strong all the other basses can't bring my health down besides Selotronprimes spin move. Other than them taking a while to bring there health down they kind of suck. I have a lot of things that are PC only content though like the S.D.M.G.
My favourite is Skeletron prime cause i have a INCREDIBLE amount of weapons. 1. my other player hits skeletron prime with the golden shower then i charge in and use my terra blade and mushroom spear and we defeat im pretty much alot now :D
Probably WoF, oddly enough, becuse his battle style completely changes the gameplay mechanics on the player. He's only fought in hell, it's a side-scrolling battle, that you need epic parkour skills for(unless you have a hellbridge, like me:3), he has three different hit boxes, and time of day doesn't matter. Plus, he's the portal from easy mode to hard mode, and he has some cool drops, like the Breaker Blade and Clockwork Assault Rifle(both of which I'm still trying to get in my current world:3). And lastly, let's be honest, he nees the guide for a sacrifice...that's reasoning enough, am I right? Lol.
By Battle: Lunatic cultist. He/she Is so fun to fight! His/her constant teleporting and such makes it a fun challenge! All those cool attacks And Trickery...
By Aftermath: Skeletron. SKT is really the first "Hard" boss you fight in the game. His battle is quite fun tbh. But once you defeat him, you are given access to the dungeon. The dungeon is a rather "enchanting" place. Serving as a place you will need to head many times in order to progress. Here is also the gatekey to hard mode. The katana is obtainoble, and much more in hardmode/post hardmode. While Not as touching as the aftermath of the WOF, i still shall put him here.