I like Swap-Bot
I think that Swap-bot is the best because it offers ATC groups along with those for many other swapping media - altered tags, altered playing cards, binder trading cards, twinchies and inchies, altered Rolodex cards, and numerous other crafting methods.
I quickly discovered which groups involved swappers that I wanted to be exchanging with - those who don't just slap together a few items and get it to the mail. I then discovered groups that required samples of my work on Flickr or a web page in order to be accepted. Once I found those, my art skills started improving immensely from the items I received, and from reading online info and books about ATCs and other paper art creations. If I found myself getting some low quality work, I knew enough to get out of that group. The group hosts are very careful about who is let in and who needs to go, and they get the word out to other groups about these members.