That's a bit of a tricky question for me; I'd have to say I-War (aka Independence War, 1 & 2). It's a pseudo-realistic space sim with a huge universe, trading, Newtonian physics etc. But the thing is that both these games are rather inaccessible if you don't have a good joystick, preferably with a separate throttle with a lot of extra axes and an (un-)healthy dedication to space sci-fi.
But I have something invested in I-War, especially the second one (I-War 2, aka Independence war: Edge of Chaos). I've tinkered with it, writing some simple mods and have tried out most of the available mods for the game. I've spent a lot more time with other games, but I haven't created content to the same extent for any other game.
It's far from perfect; if I take a step back and think about it I can find so many faults with the game, but if someone rips I-War 2 I'm compelled to speak up because I'm emotionally invested in the thing. So that qualifies, I think. That being said, I-War 2 is far from my favourite game. Besides, the scripting language for I-War 2 was really godawful; as a computer scientist (and not a game programmer) I just ended up trying to hex-hack the libraries and plug in my own compiler. That's illegal if you distribute it, so that wasn't going anywhere....