Tribes, oh god how I loved that game. I got it (and a shiny top o' the line PC) the last summer before I entered the workforce and I was determined to spend the entire summer playing the game. I probably put 60 hours a week into that thing for three months straight and became frighteningly good at it.
Mechwarrior IV is a close second. I never got to the point where I utterly dominated most every game like I did in Tribes, instead I would spend hours trying to tweak a mech so that it was perfect for a given map and scenario. I was always a fan of using a variety of weapon systems, which ran contrary to the beliefs of my compatriots who favored large numbers of the same type of weapon (The ER Large Laser being the staple of the day). In a world of mechs sporting the capacity to deliver more than 100 long ranged missiles in a single volley or unless the fury of a hurricane with a broadside of PPC's, I would run about using 2 - 4 different weapon systems at the same time. From my relatively swift (and free from heat problems) Timber wolf (2x LRM 15, 4x ER large lasers, the standard issue version most seen sported 6 ER large lasers and all mobility was sacrificed for armor and heatsinks), to my Cauldron Born that well have been the most annoying thing I've contributed to a game (2x U/AC 2, 2x Er Large Lasers, 2x LRM 15 - it had a low alpha strike potential but I could fight with high efficiency 25 - 50% farther than most of my foes, and at that range the fusillade of autocannon slugs and low profile of my mech ensured my target would likely never get to land a hit in return) I was constantly tinkering with my mechs to make them all the better. While most decried my moves as heresy, as the game expansions continued to drop I gained quite the following in the community for innovative and brutally effective designs. By the time I stopped playing 4 of the top 5 most downloaded mech designs came from my stable (including one that just baffled me. It was an Atlas design that seemed like the most obvious possible choice for the mech, and even then it was still outclassed by most other assault mechs.)
Perhaps my proudest achievement in the game was when I finally designed a mech that was fully capable of breaking out of the terrible stalemate that city combat would degrade into. Normally, players carried absurd numbers of the atrociously powerful and painfully short ranged LBX autocannon, and while they had the capacity to deliver unrivaled punishment at point blank range (200m), battles would ultimately result in 1:1 kill/death ratios for the players. I was seemingly the first to recognize that one could not meet fire with fire in this case, and came to the conclusion that even though short ranges were enforced in the level, one need not rely on knife fighting ranged weaponry. At this point, the final content had dropped, and it included three new items that would aid me in my goal - the Heavy Gauss Rifle, the U/AC-10 and the Highlander (a 90 ton jump capable assault mech). The highlander was chosen because of how nimble it was, not only could it jump it also sported the fastest turn rate of any mech in the game, and just as important, it allowed me to fit the variety of weapons I had in mind. To that end I found that a U/AC-10, a battery of 3 large lasers (less damaging and shorter ranged but they build up far less heat meaning you can keep a high rate of fire up) and a single heavy gauss rifle fit the bill. These weapons were installed in such a way that the loss of any given body area would rob me of no more than 1/3 of my firepower. In spite of the fact that it sported only 68% of the alpha strike potential of my peers, the simple fact that I could start a fight at 600m and reached maximum effectiveness at 400m meant that I actually had a significant edge. Just as important, the variety of weapons ensured that at any given time I had something that had just finished reloading, and every weapon group hit hard enough to ensure that I could keep my target on his toes. The high mobility meant that I was often able to quickly bring down an opponent before he even had a chance to effectively return fire. Suddenly, as a slightly above average player who had never achieved more than 1.2 k/d in a match in a city environment, I was achieving unheard of ratios of more than 3 against, both in open matches and clan battles. Sadly, no one ever truly believed in the concept besides me, which may have been for the better in the long run. As the only person piloting something besides a Daishi loaded down with anti-mech shotguns, I kept a substantial advantage in city maps for the rest of the time I spent in the game.