This is actually a really hard list to put together. There have been a ton of great games throughout the years, but what makes a "must play" list? The best I can do is to list out those games that are memorable and a quick "why". If anyone has differing opinions, they can do their own list.
I'll break this down into genres. Ask questions you might have about any titles. Searches of the forum will probably turn up their reviews. Oh, and I'm sticking to PC games here.
FPS -
The entire Half-Life series (Half-Life, HL2, HL2: Episode 1, HL2: Ep2, any HL expansion, and a lot of the user created mods) - The best FPS series and a great ongoing storyline. They push the envelope in story telling and technology.
FarCry - The start of the "Cry" series. Still a bit on the simple side in terms of story, but this game was gorgeous.
Crysis - The current flag bearer in graphics and a very decent entry into the world of storytelling in an FPS.
F.E.A.R. - A bit repetitive at times, but a very good game and the beginning for a new franchise.
Bioshock - Atmosphere and storytelling.
Portal - Just a great game with a lot of humor.
I could recommend more like the DOOM series, Serious Sam (NOT SS2), System Shock 2 (the original horror FPS), the No One Lives Forever series (way underrated), etc., but the above are the most current titles. (I have yet to play FarCry 2 due to the DRM shit.)
RPG -
Oblivion - The latest entry in the Elder Scrolls series. You could spend months playing this game.
The Witcher - If you're an adult and enjoy RPGs, I cannot recommend this game enough.
Again, those are the latest ones that I've played that I think will rank on any "best of all time" list. If you want to go more old school there's the A Bard's Tale series, Wasteland, the Fallout series, etc.
RTS -
Supreme Commander - The sequel to Total Annihilation and what peeps here are currently playing.
Truthfully, Total Annihilation, Red Alert 2, Homeworld, and Homeworld 2 were the ones that immediately came to mind. The C&C series is huge, but I think their latest entries have been crappy due to a camera that's positioned approximately 3 inches above the battlefield, but can be scrolled all the way out to 6 inches.
Adventure -
The Sam and Max series - The only adventure series currently running with monthly episodes.
The original Sam & Max game, Maniac Mansion, The Day of the Tentacle, Zack McCracken, Grim Fandango, or any LucasArts adventure title from the 80s and 90s are all good games, even today.
That's all I can think of at the moment. I'm sure I'll end up editing this later.
