t Chess is the one I bought. It's stable, looks good, has an easy to use interface and has all the features I need. I'm a new and fairly weak player so I need to leave it to better and more experienced players to comment on the quality of play although it seems to be well regarded in that respect.
The fact that it is a universal app (also runs on the iPhone) was a big plus (no pun intended) for me.
t Chess lite is sufficient for most uses I think. I'm a 1800 Elo player and its 1200 rating is, strangely enough, sufficient for a quick competitive game.
What we need though on the iPad is a FICS/ICC client. I haven't seen any so far.