Old school FPS games that started in the days of DOS such as Doom, Duke Nukem 3D, Quake, etc. which are all MOD-friendly.
I also like to play many fighting games in the past, typically the 2D fighting games. But I haven't really got back to them with only exceptions for Mortal Kombat as I like the latest one for including guest characters like Jason Voorhees, the Predator and the Alien, etc.
I would have voted for the Elder Scrolls and Deus Ex series if it were here, so I could have chosen 'other', but I decided to choose Halo out of that list, and I have never even played it while I have played some of the others. I just like the lore, story, feel, music and everything about halo - I've watched playthroughs of all 5 games + the others, watched most animated shorts and movies, and even read a book or two.