I would rate them in the following order

1. What.cd
2. Waffles
3. Pedros
4. Exigo

what.cd beats them all when it comes to number of torrents. Plus their staff pics and volumes are really good and you get to learn about real good music from them. However what.cd is quiet difficult to maintain a ratio unless you upload stuff. Second in comes waffles. Waffles is also good and is moderately difficult to maintain a ratio. If you thought waffles was bad then what.cd is the worst. Waffles has everything when it comes to mainstream music but when it comes to other labels and independent bands what.cd beats them big time. Pedros is known for giving perfect flacs with good covers (including complete booklets). Pedros comes in very handy in case you want one of your favorite albums with covers, booklets etc. More usefull to create a back up copy in my opinion. Their unique seeding system makes it easy to survive on that tracker. Exigo is low on content and definitely would rank below all of them according to me.

Thats just my opinion