There were alot of bosses in Final Fantasy X that were harder than Seymour in Gagazet.
The only boss that took me a long time to beat was Braska's Final Aeon. He was very hard for me. i haven't got to the end of some games though so there are probably harder. I almost got to the end of Final Fantasy V but I went into the N-Zone too soon and after beating a boss or two (I was leveled up alot) found out I didn't stand a chance against the next bosses.So I never got to fight X-Death at the end.