Mine has always been the Flect (see my avatar), there's just something about him I like. I think it's his goofy look and the fact that he's not really directly harmful to keen but indirectly. I also find it funny that he can't be stunned from the sides since his teeth unintentionally act as a shield.
When I was little I used to play with him and get him to deflect keens shots right onto a little running blooglet, stunning it

My second favorite keen creature was probably the Korath
