You want the game of the year edition, or the platinum edition. The difference between the two being that with GOTY you have to download a big patch and then whatever mods you want, and with the platinum you'll it'll come patched and include all the biggest mods, and the expansion pack.
So with one you are paying less ($30), downloading a bunch of things, and just getting the original as far as single player goes.
With the other you are paying more ($65), not downloading anything except maybe a tiny patch or two, and getting original HL single player (won many many many awards), OpFor single player and multiplayer, as well as a few training missions that don't normally come with the mods.
EDIT: Balt, add in Q3 and UT to that total and CS still far exceeds it with number of players
