For $20 or more:
The Bureau: XCOM Declassified
XCOM Enemy Unknown
The Darkness II
Bioshock
Bioshock 2
Bioshock Infinite
Mafia II
Spec Ops: The Line
The Bioshock games alone are worth way more than $20
Great! Those would be perfect for my Game Room. 
Base Tier (Pay a buck or more)
Bioshock
The Bureau: XCom Declassified
Darkness II
Beat the Average (currently $6.82)
Bioshock II
Spec Ops: the Line
Mafia II
Mystery game TBA
$20 Tier
Bioshock Infinite
XCom: Enemy Unknown
You get all the games in any lower tiers as well. If you own none of it, the $20 tier might be worth it. The buck tier is obviously worth it even if you only want one of the three games. You get separate Steam keys for each game so you can give away the “extras” in a tier if you already own them.