I have pretty much played them all to level 30ish, and then quit.
I am currently playing Castle Age and Warstorm.
Castle Age is a game where you and your friends team up to defeat Monsters and battle other players and other guilds.
Warstorm is a card game, similar to a very very simple version of Magic the Gathering. You collect cards as you level and buy new packs with the gold you make from winning.