Killzone, Call of Duty, or other online opinions?

[Kenny]

Hey guys... and girls... I'm looking for a new game to play online for PS2.

Here's a quick list of online games I already have.
Socom II: Us Navy Seals
Ratchet and Clank: Up Your Aresonal
WWE Smackdown vs Raw
Mortal Kombat: Deception

Out of all of them I'd have to say I like Socom the most. I like the idea of being on a team of people. What I'd really like is a good co-op online game where I can team up with several people online to accomplish computer missions.

I played Syphon Filter: The Omega Strain but I wasn't too thrilled with the online play. It was cool, but only if everyone know's what they're doing.

If anyone has played Killzone or Call of Duty, I'd really appreciate some feedback on it. I'd also appreciate some suggestions on other online games I might be interested in.

Thanks in advance.