It depends. If you want to spend money, twidroid pro is best- has the smoothest interface IMO. If you want free, Seesmic is best- it has all the features of Twidroid Pro (video links, link shorteners, pic link, multiple accts) but without the price tag.
What I would suggest is download the lite version of twidroid and other twit apps and seesmic and see which one fits your needs. That's what I did and stuck with seesmic. (although I'm secretly wishing tweetdeck hurries up and comes to android already)