It's hard to tell what the best campaign setting is.
I usually use either the Scarred Lands or the Forgotten Realms. Both have a lot of products (some good, some bad and some excellent).
I think that when Iron Kingdoms by Privateer and Red Spire's books come out, we'll see some more intense competition heating up the place. In addition, there's still Warlords of the Accordlands from AEG. Not to mention Wizard's new setting.
It's a good time to be a GM in search of options but it's a bad time to be a guy like me who enjoys reading about campaign settings even when he doesn't use 'em all. That's okay though. I've taken a lot of elements from SL and put 'em into FR. For example, in the Ride, I have the barbarians follow Tempus but under his more bloody aspect with a very savage leader who wields two axes...