Flight Sims are kicking some *** in this one.
Fans of more modern aviation will like this one too (made comparing the top of each category):
DCS: Black Shark - 1546 (Top Sim)
Killing Floor - 1488 (Top FPS)
Dragon Age: Origins - 1245 (Top RPG)
DiRT 2 - 393 (Top Racing)
FIFA Soccer 10 - 339 (Top Sports)
Warhammer 40,000: Dawn of War II - 324 (Top Strategy)
Assassin's Creed 2 - 250 (Top Console)
It's almost unbelievable! Only counting games that were already released, the flight simulation community managed to beat every single niche, and by a considerable distance!
Way to go, guys. Storm of War isn't in the top because it wasn't released yet, but we'll see some decent numbers in 2010

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