Mostly agree with Lacrymas, but I think you're missing a key component. They should be fun! And ultimately, that's the best thing to keep in mind in turn-based fights in general. It is insanely hard to design legitimately challenging fights in a turn-based RPG, with how difficult AIs are too design, so it's probably just best to focus on making bosses unique, interesting, and exciting. Sure, they should be harder than the average fight, but entertainment is the ultimate goal in these sort of things anyway. I think boss fights should vaguely feel like a puzzle with three or four viable solutions. Not like some ridiculously obscure solution, but it should make the player feel clever for defeating the boss.
I think King Boreas in D:OS was a pretty good boss fight. I found him reasonably challenging, and the elements rotating around the room were cool, and the elemental adds felt pertinent. Maybe he was slightly cheap with his resists, but you could get rid of them by destroying the totems. There was some build up to him, though maybe there could've been a bit more. His castle was neat and the rewards were relatively decent.
I also liked the Braccus fight. It was like, "Oh what! I have to fight all these other bosses again?!" Pretty easy to abuse the door chokepoint, and you could teleport the other bosses into the fire boss's lava death spill, but if those were fixed, I think it'd be a pretty good fight. Maybe could've been better loot though. His axe was good I guess, but I think there should've been an item that anyone could use.
