I'm using Tweetie right now. I started using that because I fell in love with their iPhone app. (I use the free version. I'm not sure I understand how their desktop version can be so much more expensive than the iPhone app, when it doesn't do much, if anything, more.)
I used Twitterlex for about a day. That day sucked. It sat on my dashboard, blank, and never connected.
If you're looking for something bare bones, Twitterific would fit the need.
I had trouble with Tweetie signing me in whenever I opened the app, and their customer service sucks. I use Echofon (formerly Twitterfon), and they've really bumped up the features in the latest release. I prefer it to Tweetie, and it takes up less real estate on the screen. The only thing it's missing is the menubar icon..
Tweetie all the way. but one little thing it needs is a number for notifications in that little red badge. for multiple notifications there is no number 🙁.
btw it might seem a bit limiting at first, but you can do everything that you can on the web with Tweetie.