I've briefly played CoD1 and CoD2 and bought CoD4 a few months back.  I grew tired of them all for various reasons pretty quickly.  The campaigns aren't really that inspiring.  Multiplayer (at least in CoD4) can be a trip for someone on ADHD, even on the hardcore servers (but you'll just find more camping on HC servers IMO).
But I'm biased on Day of Defeat (not source) since I played that competitively for about 5 years.