I never used the GF's all that often. I'll go from my personal experience over who I think is more powerful. This'll be tough.

1. Quezacotl - ~ Card Mod 4ever ~ Spamming Thunder Storm against Fastitocalon-f's for 3 AP each was handy too.

2. Cerberus - The GF that is essential. Summon him at the start and you are half invincible. Great abilities as well.

3. Diablos - Mug!

4. Eden, Doomtrain, Bahamut, Pandemonia, Alexander, Cactuar, Tonberry - Superb abilities.

5. Brothers - Defend command for Omega Weapon.

6. Siren - L-Mag refine. Silence'd mob is helpful too.

7. Shiva - Came in very handy at the start in the Fire Cavern.

8. Ifrit - Used him on ice type enemies at the start.

9. Leviathan - Recover. Offered little else for me.

10. Carbuncle - Insta-reflect was nice for a few fights.

I don't think I used the rest.