Trackmania Sunrise was the best game of 2005, for my money.
And it's not only a ridiculously fun game, but the engineering is superb too -- gorgeous looks and excellent performance even on a 1GHz CPU and GF4Ti.
And the customer support from Nadeo is second to none.
StarForce arguments aside, you can't go wrong with the Trackmania series.
edit: crap. I've just been reading another thead where someone was accused of being a shill, and have come to the depressing realisation that it's difficult to genuinely gush about a game without it looking like that nowadays -- my post looks just like an advert. sigh. This comment was edited on Jul 2, 17:29.