*SPOILERS*


Kefka's the easiest. His tiers are actually harder than he, but they're also easy. Besides, you have 12 guys against him. I usually kill him with one hit, but it's not a long battle even if you don't have all the greatest equipment.

I'd say Yu Yevon was even easier (Zombie Attack + Phoenix Down instantly kills him), but the battle against him is more like a story battle than a boss battle. You can't lose that battle (unless you're stupid enough to petrify all your own guys) so IMHO, considering him the last boss of FFX is almost the same as considering the scene when Cloud omnislashes Sephiroth to death to be FF7's final battle. IMHO, Braska's Final Aeon is FFX's last boss and Safer Sephiroth is FF7's.

EDIT: I forgot to mention. FF2 and FF:MQ's last bosses are the easiest final enemies if you use a cheap little trick against them.