I used Odin a lot when I didn't feel like fighting anything.
Other than that I used Bahamut the most and Leviathan occasionally. The few times Eiko was in my party I'd use Fenrir but I don't really have her around, ever. I think I used Madeen like once, don't really remember though.
It's not that I didn't like the Eidolons, I just had better things to do than summon them all the time.
~DragonHeart~