In thinking about what games would be in my top five, I notice that I am rather biased toward more recent games and wonder whether I should include an older game from the floppy disk era (and even the 5 1/4 inch era for those of you who are old enough to remember it).
I'd say definites for my Top 5 are:
Unreal Tournament (the original released in 1999) -- This is the greatest online multiplayer FPS game of all time. I spent years playing it, authored and released five capture-the-flag maps, managed a clan that competed on ladders, managed a custom CTF map server (TheGamebox.net CTF-2 server in case anyone remembers), joined and became active in several server, discussion forum, and IRC channel communities, and played PUG matches for many years until Sins came along. It owns my #1 spot by a huge margin. Best $10 I ever spent. I still play UT 2004's Onslaught and Invasion-RPG frequently.
Baldur's Gate series
Alpha Centauri
Empire / Empire Deluxe -- My entry for the floppy disk era.
Contenders for the other slot in my top 5 are: Star Fleet I: the War Begins (anyone remember it?), Ultima III, Ultima V, Starflight, Mechwarriors 2: Mercenaries, Quake1/2, and Neverwinter Nights, and Sins. Sins is a very strong contender for a Top 5 spot, but I'll have to see what I think about it in two or three years.