Question TF2? Orange Box, or COD4?

Thinking about getting a new shooter. I was looking at just getting the single purchase of TF2 on steam, since I donot know if the whole orange box collection is worth it. I doubt id play the other games, but I hear portal is fun. I am also hearing good things about COD4. I just want a shooter, CS:S style or Rainbox Six-ish where I can do a few quick matches if I want and bail.

Prz, halp.