Although I hammered Doom, Doom II and Quake the first game that got me to invest heavily in PC gaming was Unreal Tournament 99. As broadband wasn't available at the time I invested (heavily!) in channel bonded ISDN lines to get my ping to playable levels, I even played a few pro tournaments for the clan I was in.
No other game has even come close since.
No other game has even come close since.