For me it would be a moogle. It can do my mail or transform into something useful plus most of the times it will just float somewhere in and around your house.
A cactuar will juat run around and leave needles everywhere. With a tonberry i would be afraid to go to sleep for a knive in my back or a constant reminder of everthing i did wrong to someone. And a chocobo even if they are cool and cute they are big birds, it wont fit in my house.
