If you've got some money left over and a LOAD of time to play it, I'd go with the Steam "complete pack". You get:
Counter-Strike, Counter-Strike: Condition Zero, Counter-Strike: Source, Day of Defeat, Day of Defeat: Source, Deathmatch Classic, Half-Life, Half-Life 2, Half-Life 2: Deathmatch, Half-Life 2: Episode One, Half-Life 2: Episode Two, Half-Life 2: Lost Coast, Half-Life Deathmatch: Source, Half-Life: Blue Shift, Half-Life: Opposing Force, Half-Life: Source, Left 4 Dead, Peggle Extreme, Portal, Ricochet, Team Fortress 2, and Team Fortress Classic.
for $99.99 on Steam. Yep, that's 25 games for $99.99 dollars, meaning just $4 per game. Add to this the L4D Survival pack coming out in mid april, and you're onto a winner.
On a side note, what are your specs?