I used to not like Cactuar very much, because the damage was low, but now he's doing 10000 to all, so my respect has heightened muchly.

Probably leviathan. The summon sequence is nice, but it's too long to not be annoying, and not long enough to be too useful to boost. Plus the only time you really need water attacks is when you're fighting cactuars, and then it's just easier to fight (since they run away uber fast)

probably a second one would be siren. she's obsolete when doomtrain comes onto the scene, and I'm not all the big about wannabe mermaids.

I really liked carbuncle The little guy is too cute to not^_^ (and I've got moderately good memories of the one from ff5)