Hi Pioneer,
"Best" may be subjective in this case, Method Mobile works great on a lot of available smartphones. I personally use it on both the iPod Touch/Blackberry Torch and it works great using the Mobile version of Method. If your question is in regards to the new tablet devices that are popping up everywhere, there currently isn't a version of Method specifically for tablet device though the mobile version should still work(may look a bit off) but I see the non-mobile version of Method working on those devices as those devices improve. You may also want to take a look at this blog entry here, it goes over some of the recent improvements to Method Mobile
John- Maybe the question is just in the title.
-Michael