Realism with a capital R.
As much as Doom was my gateway into computer games and its rich potential, Rainbow Six and its sequel Rogue Spear were my meat and potatoes. Back when I actually had a subscription to a gaming magazine (God, those were the days...) this preview for a game based on a Tom Clancy novel immediately caught my attention. I had gotten plenty bored of Quake 2 and was restless for something different and unique. The ultra-realistic gameplay of Rainbow Six fit to a T. Still, my computer at that time was unable to meet the minimum requirements for R6, so it would take a couple months before I actually got into it. But get into it I did, with a passion that now seems somewhat inhuman. I lovingly outfitted each operative with different weapons, carefully mapped out their mission routes and held my breath as they soldiered out to take down the terrorists and rescue the hapless hostages. Of course, on more then one occasion the entire operation would turn into a fiasco with multiple casualties and hostages gunned down. Alas, back to the drawing board.
But multiplayer was where Rainbow Six really shined. Freed of the cumbersome AI, it was amazing to encounter a situation where you could be taken out by the opposing team in just one or two bullets. The added lethality of the weapons and the reduced margin of error made for the most amazingly tense multiplayer matches ever. Rogue Spear, the sequel which I bought on opening day was much more of the same. I have to be honest, Rogue Spear multiplayer devoured my life throughout 4 years of high school and doubtless contributed to the fact that my GPA suffered and I never had a girlfriend. Oh well, it seemed worth it, that's just how obsessed I was with the game. Even outside the game itself, I posted frequently on RSE's (the maker of R6) forums and made quite a few memorable friends, who later formed their own personal forum at planb3.com. Rainbow Six is truly an immense chunk of my life and I have nothing but fond memories to look back on.