The ones I'd use the most would be Bahamut, because of his powerful attacks and Doomtrain.

IMO, Doomtrain was one of the best. When summonsed, he would inflict all the abnormal statuses the current enemies where weak against plus he would cut every enemy, including bosses vitality to Zero! Also, I liked the fact that he came with ST-Atk-J and Elem-Atk-J already learned, plus he was able to learn ST-Def-Jx4 and Elem-Def-Jx4 for a low 180 points each.