Get the CS:S, DoD:S, and HL2-DM package. They sell that one retail for $20 or 30 (can't remember) too. With any one of those you should be able to play any of the mods like Insurgency or Goldeneye:Source.
I don't really play much CS:S anymore, but the type of people in DoD:S, HL2-DM, and now Insurgency are no worse than any other games I play.
I would personally stick with a Source package as that is where the newest mods are at (in addition to the above mentioned, stuff like Pirates,Knights,Vikings is cool) and I cannot stand the old HL-based games. Yeah back when CS was in early beta it was the bomb, DoD as well, but now I just can't do it. I even tried giving TFC yet another chance as Quake TF was probably my favorite multiplayer FPS ever, but just couldn't get into it like I used to.
Unless you want to pay the $50-60 for the complete package, then you have everything. I have the old Steam Silver package for $60 in 2004, and even though I don't care much about any of the pre-HL2 games anymore, they were all bonus anyway as what source games I got for that $60 are EASILY worth it alone.