Oh, this was such a good year, so it feels like somewhat of injustice whatever I vote for.
But...Sid Meier's Pirates! is the game I played the most and that I find to be best from that year, along with being quite influential. Open world game with open ended gameplay, leaving you to choose what will you do in your life as a pirate/buccaneer/trader, and what nation(s) will you join or plunder, all the while ingame clock is ticking to your eventual retirement. Really surprised not to see it in main poll.