The summon spells in Final Fantasy VII were simple, caused a moderate amount of damage, and were short enough so as not to slow the game down when you summoned them (with the exception of Knights of the Round.) I still remember when Bahamut ZERO was first summoned. I thought "Wow, a dragon in space." I was a little biased I suppose.
The idea behind Aeons in Final Fantasy X was also pretty sweet. A fighting summon spell, who had its own limit guage for optimum damage. A neat idea, even if it took your party out of the fight.
The thing I liked about FFVII and FFVIII’s summons were that you can equip anyone with a summon whereas in FFVI, FFIX and FFX you have specific summoners.
I prefer Aeons, it they were just awesome, especially since you could upgrade them from Yuna’s Sphere Grid, not to mention how much you could customize them… the idea of them fighting was awesome, but I think they made the game way to easy (Anima, and the Magus Sisters O_o)…
I also like the Espers of Final Fantasy VI, you could only equip one per character, so you couldn’t summon them over and over again, plus you needed them to learn your magic spells from them, I thought that was in interesting concept, it turned finding Espers into more of a hunt for magic spells rather then summons…