At your level? Let's see...

Combat:
-Dire lion
-Dire boar
-Polar bear
-Rhinoceros

Flying:
-Dire bat (if in combat)
-Eagle or hawk (if you want more speed)

Aquatic:
-Giant octopus
-Orca (in 1 level, anyway)

Other: Anything inconspicuous is a good choice for sneaking around. Things like cats and owls don't attract a lot of notice. By contrast, the Huge viper isn't a very good combat form, but turning into a 30-foot snake is fun just for the shock value, really...