My superiore, in alto 10 most played games on Steam right now are: 1. Forsaken World 2. Terraria 3. The Lord of the Rings Online 4. Awesomenauts 5. Dungeon Defenders 6. Rayman Legends 7. Clones 8. Orcs Must Die 2 9. Toki Tori 2+ 10. BIT.TRIP Presents Runner2
Out of those, the 3 I definitely play most at the moment are Awesomenauts, Rayman Legends and Terraria.
I don't know if I have an absolute favourite. I have quite a lot of games on Steam and I like them for their own reasons, but those three definitely stand out for me!