I liked Peep, it just didn't have enough features for me.
Tweetcaster was nice, feature packed and all, but I thought the app itself was kind of ugly.
I also refuse to pay for a Twitter app, especially when it's--for the most part, unnecessary.
Which brings me to Twitter for Android. I didn't care much for the new style of Retweeting, which I took care of by downloading an app that works in cohesion with Twitter for Android, simply called "ReTweet".
It allows you to share a tweet with it, all while within the official app, and retweet the old way.
Not only that, but the official app is pretty feature packed and it's free. It also looks nice.
If I had to rank based on my uses, I'd go:
Twitter for Android
Peep
Tweetcaster
Touiter
Seesmic
Twidroid.
Of course, the last 3 I only tried the free versions.