Twicca, is the best one for me. I am quite happy with the UI, all the fancy looking UI's are great for showing off your device, but I really just want to read the tweets and having a clean none cluttered UI like Twicca is perfect for me.
The only thing that I don't like is that it does not remember your position in the timeline if you refresh or go in from a notification.
I use Vodafone Update. It lets me view/update Twitter and Facebook with one app. Coming from a BB it's not a good as socialscope, but the best I've found on Android so far.