Well... To name a few:
Pokemon Crystal and Sapphire. Over 250 hours logged in each. I was 7 and 9 respectively, had no friends, and thus a lot of free time.
Mass Effect series. Do I really have to explain this one?
StarCraft series. The past 15 have been good to me.
WarCraft series. The past 19 have also been good to me.
PlanetSide 2. 12,000 people per server and 4,000 per continent, the developers are very active in the community with the Roadmap feature, and it's free.
Natural Selection 2. It's what AVP 2010 and Colonial Marines should have been.