That would depend on what you liked. I like shooting people so for single player action I would go for Half Life 2. If I want something a bit more in depth or thought provoking I will hit up Fallout 3 or Dragon Age: Origins.
Sometimes you can't beat a bit of Command & Conquer.
Monster Rancher 1-3
Legend Of Mana (Can also be 2 player)
Conker's Bad Fur Day (The Original)
TES Morrowind. (But it gets lonely if you are used to online games.)
Fable was always fun.
Black and White (1 and 2)
I don't play many games anymore. =( I wish I could but I don't own a system or anything. And I do not have a job so I can not buy games! =(
If you like puzzle games, there's The 7th Guest and The 11th Hour. Though, be prepared to spend a very, very long time on them. Unless you're a puzzle wiz. In which case, give me your brains.
The original Metal Gear on the MSX is alright. Dated, but still fun. Make sure it's the MSX version though, the NES? Pretty shit. To the point where Kojima doesn't really consider it Metal Gear. Though, if you want an improved experience, go for Metal Gear: Solid Snake (MSX again). Adds new features.
For adventuring, there's the Ace Attorney series, Hotel Dusk: Room 215, the Monkey Island series (I can't comment on Tales of MI), and hell, let's throw the Quest for Glory series in there as well.
I can't think of anything else. Because otherwise I'd end up listing a hell of a lot more, and at this point, the requirements are vague, so...I'd be here forever.