To start off: it's a perennial favorite that I tend to re-install every other year: [Open Transport Tycoon Deluxe](https://www.openttd.org/), a clone (and expansion) of a game of the same name by the legendary Chris Sawyer.

Bonus points, the graphics and sound have been completely redone meaning the entire game is free - no need for the original game data files.

For me, FreedroidRPG is one of the best, also SuperTuxKart.

Xonotic is one of my favourites. Don't play it a lot but I enjoy it when I do. Online multiplayer is quite brutal though since the few people that play the game seem to pretty much have mastered the skills. Dungeon Crawl: Stone Soup is also a mighty fine game if you like roguelikes (in the true meaning of the word).

Wesnoth is my single most favourite opensource game project. Great game, not a clone, nice single player campaigns, and fun multiplayer as well.

Because I also love theme hospital I'd say corsixth is nice as well, but you need the original data files. It is a worse than the original in some ways, but high resolution and not needing dosbox is nice.

If I'm going by hours played (invested), it'd have to be OpenTTD. Otherwise, it's a toss-up between OpenMW and 0 A.D.