Command & Conquer: Tiberian Dawn (the first one)
There are.. frankly more reasons I think this game was one of, if not the most revolutionary game in the industry. If memory serves it was the first real RTS, so there is a good point right there, in addition it was the first game to have tactical action going on between two sides that were actually different. Sure, rescource gathering was a little bit simplistic, but the units and strategies employed by either the Global Defense Initiative or NOD were truly diverse, and there were a couple of uber-strats rather than there being just one, so even when the game was years old there was still diversity. I could be building a mass of differing tanks and then go storming into my NOD opponent's base only to be swarmed by massive numbers of cycles that picked off half my tanks before being squashed themselves.
Wow. Off-track much?
..Anyway.. That is what I believe was the most revolutionary game out there, it redefined the gaming industry even with just the onslaught of clones that came afterward.. it was not perfect but C&C is one of the most fun games out there even now. Probably could be considered lame reasons, but gimme a break, I haven't had my coffee yet.