Ok, so follow me on this one...
My favorite is Touiteur. Has just about everything I'm looking for in a twitter client. There are a few things missing but I can live without them. The only thing I wish was better were the soft buttons at the bottom. I wish the dev would spice em up a but and give it some pizazz.
With that said, I think the BEST twitter client (and my runner up) is Twidroid. It has everything. Albeit slow at times and maybe not the best layout, it packs everything's into the app. You don't have to switch between clients because one has one function the other doesn't. Twidroid has it all!
And third, its a tie for me between Seesmic and Tweecaster. Seesmic look, ehhh...good. But falls short in 2 critical areas for me. 1, you have to go into menu to compose a tweet! That's horse manuer and completely unacceptable. 2, there's no jump to top button. So while it remembers your last position and if you're like me and sometimes takes 3 day breaks from twitter, you have to scroll ALL the way up to the top on your own. Being able to share tweets with facebook is cool, but for me (and this is an opinionated post) what I post on twitter I usually only want my twitter friends to see it. I use facebook more for networking and family so I like to keep it clean on there. As far as Tweetcaster goes, this app is underachieving big time. It COULD be SOOOOOO much more. But its not. It looks exactly how I want my twitter client to look but it doesn't behave that way. No lists, no threaded convos, notifications are buggy, etc etc. If the dev could just tweak these minor things to get them to work then it would be without a doubt the best and my favorite twitter app. But because of the aforementioned problems and many others it ranks 3rd.