Forgotten Realms is clearly the richest and most detailed and maybe the favorite by WotC but this could cause a little fed up, like a superpopular song that becomes "old-fashioned".
Dark Sun is the freshest and most original setting with its own style but it is like incomplete, the metaplot couldn't be ended and today the banning of classes and playable species could be a serious handicap.
Ravenloft is cool but the dark domains are too "small" to allow space for conflicts between supernatural factions.