Keep in mind I'm not actually going to SUMMON any of them... I never summon in this game, but Junction... whoa nelly!

Siren (L-Mag Ref, Treatment even though I also use Item instead of GF)
Doomtrain (Junk Shop, ST-Def-Jx4)
Tonberry (Call Shop, Haggle, Sell High, Familiar)
Cactuar (All the bonuses d00d!)
Quezacotl (Card Mod)
Ifrit (Ammo Ref, Str Bonus)
Diablos (Mug, Time Mag Ref)
Bahamut (Str+60%, Mag+60%, Abilityx4 all for free, and Rare Item)
Eight-way-tie