After playing the PSP remake, I gotta say besides Bahamut, my favorite summon spell is Leviathan or the Mist Dragon. With Leviathan, they finally made his wave look cool and powerful. With the Mist Dragon, I like how he slowly formed from within the mist when you summoned him (or her?).

Titan was cool, too, but not my favorite. And Odin, I was disappointed to see they just re-used Ramuh's spell animation with him.