I use mainly Twimgo as it auto-updates. It's also quite Gravity like, and has some cool nice transitions, and is really good in portrait.

But it occasionally seems to go a bit bonkers for me and can tie up the processor and drain the battery.

Otherwise Tweego is also good, but does not auto-update, but does display a message on the page that there are new tweets.

Kweeteur used to be my preference, but an update removed (or hid) the direct link to URL links, and broke important functionaility for me - good otherwise and fairly fast.

I could never get Witter to authenticate for me since twitter switched authenication modes, but did not like the user interface,although that is a personal thing, and it may be a decent client.