I've used the original iPhone 3G keyboard and on my Droid X I've primarily used Swype and Swiftkey. I played with the Google keyboard but I prefer Swiftkey for its ability to sometimes guess the next word, although I do agree the keys are bit on the small side, and that's with the oversized screen on my Droid X.

Swype is an odd bird. When it works, it's genius. For three-character words, it was often quite wrong, and its dictionary was loaded with quite a bit of junk that you couldn't remove that would also tend to match on short words. On long words, if you screwed up half way, you'd have to start all over again, which really sucked. Also, if you just washed your hands, then your fingers wouldn't slide smoothly on the screen.