My serious money leagues are on RTSports. I have a couple Yahoo leagues, too. Yahoo is great for the basic league setup, but we get into divisions, etc... in my dynasty league and keeper leagues so we needed to use a pay site for those. I've been very happy with both.
I used CBS and ESPN's sites a few years ago and hated them. CBS was ok as far as dependability but I didn't like the interface. ESPN is just crap...and that's when you can get connected at all.