Here's what you do.
Get Half-Life 2 through the
Source Premier Pack. (Or if you want Half-Life also and all it's mods:
http://storefront.steampowered.com/v2/i ... &SubId=81& for the Complete Pack) That way you get HL2, EP1, CS:S, DOD:S and can also use all the kewl Half-Life 2 mods, plus you get all those other Half-Life 2 based games.
Counter-Strike: Source
Day of Defeat: Source
Half-Life 2
Half-Life 2: Deathmatch
Half-Life 2: Episode One
Half-Life 2: Lost Coast
Half-Life Deathmatch: Source
Half-Life: Source
If you just buy Half-Life 2, you DO NOT get Counter-Strike: Source or any of the other things. Just Half-Life 2 and the Lost Coast technology demo. Considering all of the other parts seperately would cost $20 each, it's worth it to spend the extra $10 to get the bigger pack as you might decide you want them later and you'd have to buy them all seperately. Not kewl.
After that, then you can consider Call of Duty 2, F.E.A.R., Condemed: Criminal Origins, and FarCry for single player. Though their single players probably aren't as deep as HL2. And their multiplayer functions pale in comparison a ton.
Battlefield 2 has dropped off because apparently EA has broken it with a new patch. And they've never fixed any of the major bugs, mainly ones nobody noticed instead.