I'd say VIII has the best looking summons I've seen. I've even collected most of the GF action figures (except Shiva and I'm not sure if they ever issued Quetzecoatl). (I haven't played X, but I thought the action figure for Anima looks more evil than any of the monsters!).

I really liked the fact in IX that they used weapons most of the time instead of the full summons. Full summons chew up a huge amount of game time and although the power was lower, I liked to be able to keep battles shorter.

That being said, I think that the most overblown summon spell is definately Knights of the Round. And am I the only one who sings the 'Camelot' song from Monty Python and Holy Grail when summoning KOR?