ozma (ff9) is the most fun. dont fight him at the end game, try beat him earlier, as soon as you can get to him

ultima weapon from ff8 is kinda easy but if you're trying to draw ultimas he's hard

omega weapon from ff8 is a decent challenge if you're not cheating by using hero drinks/certain limit breaks

emerald weapon without resorting to cheese methods is a decent challenge (knights of round spam, cait sith etc)

ruby weapon without resorting to cheese can also be sort of challenging (if you havent spammed sources to get OP)

ffx dark aeons and the arena final bosses/penance are probably the hardest. try to beat them all without yojimbo and wakka

all pretty easy at the end of the day, but as a "normal" well adjusted human who isn't a sperg or sweat, those things should be a fun challenge

funny thing is we havent even gotten close or anywhere near how much fun turn based could be, the world just became retarded ASF and stopped making turned based games