As in a scene with a summon? Or the cutscene that plays when you summon?

If it's the first, then FFIX, Battle of the Iifa Tree, with Bahamut.

For the second, gotta go with Shiva from FFX. I love that intro so much.

A very odd one i will admit but i really like Carbuncle's VIII animation. I love the way he puts his head out slightly before finally coming out of the hole

Any of the Bahamut summons in FFVII. Or the Tonberry summon in FFVIII.

Of course, Odin's final FFVIII summon is always notable, and when Gilgamesh takes his place.