Thumb Keyboard is quite useful, but my favorite is SwiftKey3. I use it on my 10.1 inch tablet (a Toshiba Thrive, but it works on phones as well). The keys are well placed and there are lots of ways to customize the keyboard. The best thing about it is the predictive typing option. On average I have to type about 60% as much as if I had to type every word in full. That really speeds things up.
I do wish that the number keypad in the middle of the keyboard could be edged towards one side or the other. As it is it's quite a stretch to type the numbers. Also, I wish that one of the (two) spacebars could be used to put spaces in a document while the other is used to trigger the predictive spelling. As of now it's one or the other.
--Michael