Mechwarrior 2: Mercenaries
It is easily my favorite game of all time. The sheer number of Mechs, weapons, missions, etcetera gave it a huge open-ended feel as well as it taking place at one of the most interesting points in the battletech timeline (right before and during the initial clan invasion). This gives it the advantage of being able to realistically reveal new technology as the game progressed as well as make the plot more and more epic... become commander of mercenary unit, become involved with the clans, etcetera.
Furthermore, it demonstrates incredible dedication by a company to continue to improve the game.
After the game had already been out for several months and hardware acceleration really started taking off, they released a 30+ mb patch (huge for us dialup users at the time) which added d3d/glide acceleration as well as completely new textures and effects. Not only that, they changed the salvage system from missions from being scripted (the same stuff each time you completed the mission) to dynamic (suddenly you really wanted to start getting headshots on enemy mechs...). To me this is even more significant than the 64-bit content pack and addition of HDR rendering to Far Cry.
Finally, it was just FUN. The game mechanics, the story, everything flowed well and just ended up in a truly enjoyable playing experience.
Dang, I really need to get that old PII-400mhz with dual voodoo2's running again in windows 9x so I can play it again