Dirt Rally 2 for me. It manages to somehow improve over the already excellent Dirt 1, with great, unforgiving gameplay and an unparalleled sense of speed, and the daily and weekly events are a great.
Just about the only real drawback (other than the always online requirements) is the rather small amount of content, as well as the unnecessary inclusion of Rallycross. But it just feels so good to play that both of these are easy enough to forgive.