Portal2 was great (as all Valve games are). Probably the best overall game this year IMO.

Brink is the best LAN party game of 2011.

Warhammer 40,000: Dawn of War II: Retribution is the best RTS game of the year (DoW2 is the best RTS series of all time).

Just picked up Arkham Asylum today on Steam for 4.99 so we'll see if I get into it enough to pickup Arkham City.
HT v2.0 !
HT v1.0