I am a big fan of both free, and pay and what comes with both.
For free, I suppose I would side with Yahoo since they offer a pretty good quality product, though to get any of the features (stattracker, kits, etc) each are in the $10-$15 range, which is 2/3 the price of a pay site that may offer more features inclusive in the price.
For pay sites, I love fanball.com, just because they have a wide variety of games offered, and some nice cash prizes on top of it. I had tried a few other pay sites, but I think fanball gives you the most bang for your buck in terms of what they offer you and what advice/tools are given.
There is no doubt in my mind that ESPN is way better. we switched from yahoo last year adn it is consensus amongst the 10 teams in our league that everyone likes it better. also, it keeps records from years past, which is nice. and players are always available.. you don't need to wait for that magical date when yahoo adds them and then there is a race to be on the computer at that moment to pick a player up. if you are the first to know about someone, or the first to take a risk on picking him up, he's yours. also, free live scoring...
A lot of it is familiarity. The other services are catching up but for a long time Yahoo was far and away the best fantasy provider. It is also the most stable IMHO. ESPN really crapped the bed the first year they offered free fantasy service. The system was a mess and the drafts kept crashing. So first impressions and all that good stuff.
Maine has a good swing for a pitcher but on anything that moves, he has no chance. And if it's a fastball, it has to be up in the zone. Basically, the pitcher has to hit his bat. - Mike Pelfrey
I play on Yahoo with buddies because it's free and then we pay $50 a piece to the commissioner and then pay out winners. 1st place overall, and then 1st and 2nd in playoffs get prize money.
I'm a fan of ESPN. Their stat tracker is free and works well. Enough comish control, and I like their FA page better. Yahoo isn't bad, but the fact that you have to pay for the stat tracker is criminal.