Definitely Metal Gear Solid 2, Quake 2, Summoner 2 and (mostly) Unreal Tournament - the game that finally convinced me to get UnrealEd and abuse it to the point of overheating my first laptop many, many times.
Doom, in high school about a year after it came out I learned you could change the game and make levels, which I did with a buddy of mine, we also did dial in multi-player. We had so much fun making custom maps and playing against each other. It was a pain in the ass there was no one around to ask we just had to figure it…