Korg is a good brand for this kind of thing, I think.
So I would suggest that Korg one.
I would asctually suggest something like that Bliptronic thing, but it's made by Korg.
It is also expensive, but I would find it worth it.
EDIT: I'm pretty sure what I'm talking about is a Korg Kaossilator.
But a better version of just about the same thing is the KP-3.