I think Oolo looks Asian. That sounds racist, even though I don't mean for it to.
I also thought it was hilarious when Bucha did that deep, "Keew keew."
Machi for me. Though I'll admit this might be at least partially due to the fact that he's the first one and most of the others didn't seem all that different to me. But I loved it when he was scared by Fi popping out of your sword. That was hilarious and adorable.
Ever since I first laid my eyes on Machi it was love.. Err that didn't come out right You guys know what I mean. He's pretty darn cute! Off topic though my favourite is definately Oolo.