God there's a few games I love listed on here, but I gotta give love to a few mods. They all have an equally special place in my heart.
First is Tour Of Duty mod for halflife. Was just a pure hilarious game, but I just always played the sapper class. Probably the game I was "best" at for me.
Next would be Desert Combat. An amazing mod that was fun to play for the large scale battles that would happen. It was so good my dad bought another pc so we could play together and do chopper runs dropping and transporting troops.
Thirdly would be Movie Battles 2 for Jedi Academy. Probably the most impressive mod I've played. The saber combat and blocking system was pretty damn good, and as a Star Wars nerd it's one of the best Staw Wars games. You can play as most characters, each falling under certain classes, huge maps, crazy battles and there's nothing like naming yourself NotSoAmazingPantlessWookie and tossing nades to knock guys down and then punching em to death. Simply AMAZING!!!