In a purely RPG sense, Ifrit really feels like the final boss since it makes use of all your gear/ascension/etc. of the full party, rather than a 1v1. Yeah, yeah, action game, you control Noctis, party is kinda useless yadda yadda, but Ifrit really did feel like the final fight because it was a culmination of both RPG progression as well as learning the combat system - linkstrikes, weaknesses, warpstrikes, aerial combat, parrying, knowing when to hit him (when he's not on fire) etc. etc.
The only problem is that due to the open nature of the game's progression and the rather steep leveling curve that you can bust through by doing a bunch of easy quests then cashing in 200k x3 in Altissia, a 'final boss' gets pretty hard to tune, and no fight can really be called epic when you blow through phases in about 30 secs apiece, with or without summons interrupting.
edit: Speaking of end-of-main-quest balancing, it also feels like they acknowledged it by having a smorgasbord of boss-level monsters in the streets of Insomnia, so regardless of whether you were level 45, 60, 80 or 99 you had something sorta challenging to beat up on.