Diablo fans READ THIS!

Lysergic: I'll drop you my two favorite games ever on the computer. Both are a series of games (trilogy), but I'll bold the best ones.

Marathon, Marathon 2: Durandal, Marathon Infinity (By Bungie)
Ignoring all else, this series would get best game award for the story alone, not to mention how the story is told, gameplay, multiplayer, etc.

Escape Velocity, EV Override, EV Nova (By AmbrosiaSW)
An enormous web of a game, masterfully done. Everything but the source engine is open to be modded by players, in an enormous and unbelievably skilled community that I am glad to be part of. The Aftermath TC is going to be awesome.