In FFIX, my favorite summon is a hard call. Madeen is awesome (seen it more than most 'cause Eiko was in my party throughout Pandemonium and Memoria, even though I can't seem to beat Necron...), but I've always loved Leviathan and Shiva, so they're up there too. I think those are my top three. Though, Bahamut's cool (haven't gotten Ark yet, so I can't compare), and Ifrit's nice if you need a big, hulking hell demon to clobber things that don't absorb or null Fire. Heh. But I think I'll stick with the first three: Madeen, Leviathan, and Shiva.