i've tried all of them and the best one so far for me is Doodle Droid. It does cost 99 cents, but I think thats a small price to pay to support a decent app. You can download it and try it out, if not satisfied, you can get a refund.
I'm currently waiting for Layers to be released on Android. Its already on iPhone and I've seen amazing things drawn on it. The coolest part about Layers is that you can save the files as PSD files and open them in Photoshop.