Diablos, Doomtrain & Eden, all three had great attacks and excellent learned or learnable abilities.

Bahamut, Tonberry and Quezacotl.
Bahamut: His Forbidden Magic Refinary came quite useful to all my characters. I have Ultima and Meteor maxed on them!
Tonberry: Even though i was level 100 when i got this little bugger, I loved his Level up ability, along with Familiar, Call shop, and Haggle. These three abilities come in handy when you get to Esthar and can make and SELL megapotions.
Quezacotl: Card and Card Mod. I love playing cards because i always win!
You can get really rare items from really rare cards....if you know where to find them, of course XD

so true but i only missed one person with only 83meteors :( and tonberry was useful in making that extra money while on the go
Quezacotl was eh to me.......
i guess i find Cactuar,Bahamut,and Tonberry useful so what i added two more lol >.<

Leviathan for me. When I was fighting mechanical enemies, bosses, etc, I used him like crazy. Also in the sidequest going after Jumbo Cactuar I used him a crap-load to take that prick (no pun intended) down.

EDIT for below: Woops, that's what I meant. :p

You use Quezacotl for electrical enemies!

rofl lol

Quezacotl was useful at the begining of game and when you're fighting that robot that made its way to FH.