Well, my personal list, based on games that were awesome for their time:
Half-life 1/2
Planescape
BG1-2
Freespace1-2
Fallouts
Starcontrol 2 (best space game evah!!!)
Xwing/Tie fighter
The first two Lands of Lore
Some of the original Wing Commanders (not all, some of them aren't so great)
Indiana Jones - Fate of Atlantis
Monkey Islands
Grim Fandango
Command and Conquer, and C&C: Red Alert
Dune 2 (mentioned already, but awesome)
Doom II
Kings/Space Quest, Quest for Glory series (QFG 1/2 available on www.agdinteractive.com, remade with better graphics and can run on modern machines)
SS2, Deus Ex, mechwarriors, etc etc etc etc
...
there's way too many, I don't want to fill up pages.

Duke Nukem : Forever
If you're over 30, better hope you live to 90.

Hate to be the cynic but these are all highly subjective. I mean, I'm not going to say WoW sucks and I'll never try it. Instead, I commend it for what it's accomplished and it probably is a very good game, it's just not the type that I get any enjoyment from.
However that being said, this is the only entry that's worth it! lol

Hell is having a finished copy of Duke Nukem Forever and Atari 2600.

Ofcourse, everyones list should be different I thought that was the point! I've never been too keen on point and clickers like Grim and Monkey's Island for example Both these games I have tried desperately to like. I am giving the genre another shot with feeble files but I am just not patient enough and end up getting frustrated :P

Use the Schwartz....

An excessively complicated, rivet-counting flight simulator. Preferably a modern combat one - everyone should, at least once, struggle with a radar and curse as they try to persuade a missile to leave the aircraft, or swear in a bad russian accent because it's midnight and your nvgs aren't much help in picking out virtually invisible targets on your shkval. Everyone should, at least once, spend ten, twenty minutes running through the cold start procedure on such a simulator. It's... an experience. If that doesn't sound appealing, at least try a realistic flight sim of some kind.
Also, everyone should play a roguelike - one of the older classics, not one of the more recent games. Winning is unlikely, but you'll learn something. Rogue would be good, but nethack or crawl are more modern and still classic enough in style to be worth spending half an hour on.
Once they've done that, they should play DoomRL, to learn that it is possible to convert a fast-paced FPS into a roguelike whilst keeping the general feel and atmosphere of the original. Seriously, it's scary how Doom-like it feels.
Finally, everyone should try Tranquility. It took me about five minutes for the controls to really click, and then it was wonderful.

Oh, a major game is missing: Lemmings!