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.