Final Fantasy IV
Bahamut. His animation in the 3D version is awesome, and Megaflare saved me a bunch of times.

Final Fantasy VI
Kirin. I like it's design, and I used it to teach Cure to most of the party.

Final Fantasy VII
Bahamut ZERO. Bahamut in space is just a really cool visual.

Final Fantasy X
Anima. She's absolutely terrifying and completely unforgettable.

Final Fantasy XII
Zodiark. One of my favorite optional bosses.

Final Fantasy XIII
Bahamut. The way he saves everyone while they're falling to Gran Pulse is awesome.

Final Fantasy XV
Shiva. Diamond Dust is always stunning, but it has never looked better than when it shatters Ifrit in this game.