Must-Play Games? Which are the True Gems?

Bioshock Infinite is great if you have the time to play it.

I don't, and that is kind of saddening.

Portal
FTL: Faster Than Light
SimCity 4 Deluxe
GTA San Andreas, IV

Dungeon Keeper :)

Bad Rats, Horrid Henry, Rambo the video game and SotMC are among the first games that come to mind

Portal,Alpha Protocol,F.E.A.R.,Little Inferno,Serious Sam games,Trine . For sure you got quite a collection of good games to play xD

There are TONS of good games in general, I suggest you the following from your library (just looking at the ones with no playtime):

The Binding of Isaac (although a full remake is coming in the next months)
Portal 2 (just make sure to have beaten the first one first)
Half-Life + Half-Life 2
Batman: Arkham Asylum GOTY Edition

Hotline Miami is quite fun.
Orcs must Die (original) is very fun.
Mount & Blade: Warband is super fantastic.
I noticed Portal 2, Portal (original) is a really great game. Portal 2 is completely different, but enjoyable and worth playing for a laugh.

I don't think anyone has said this yet but the S.T.A.L.K.E.R. Series is amazing. So much atmosphere and you can leave it and come back later without much consequence. I wouldn't recommend leaving during a quest, because you might forget what you're doing, but if you leave when there are no active quests (not hard to do) you should be golden.

EDIT: The series does comprise of 3 games but you could play any of them without much confusing. If you were to pick only one of them, I'd recommend Call of Pripyat.

Super Meat Boy
Spelunky
Portal 1 and 2
FTL
Deus Ex
The Witcher series
Civ IV (V if you have the DLC, but 4 is way better otherwise.)
Planescape: Torment
System Shock 2

As I see, there's no dungeon... Eye of Beholder II, Gates of Skeldal, Wizardry VII, Ishar trilogy,...

Both Batmans are really good games. Also Magicka, especially if you have someone to play it with - it's most hilarious this way! :)

Also I would reccomend Dust an Elysian Tail, it's currently in Humble Indie Bundle 11 for minimum of 1$. It's one of the best games I played.

Both Portals

Batman: AA and AC

Mark of the Ninja !!

Thank you so much for all of your replies so far, everybody. I really appreciate your input and plan to check out/play as many of these games while I still can. I appreciate you voicing your opinions for me.