For the first 3 games (COD 1, 2 and Modern Warfare) and all their expansion packs, I personally believe Infinity Ward dominated the market (actually, they didn't, but the games were all great). Treyarch's games were certainly good but just weren't as good as Infinity Ward's first productions.
However, with the release of Call of Duty Modern Warfare 2, I'm starting to lean towards Treyarch, and hopefully COD will be in their hands from now on. Overall, Infinity Ward has better titles in my opinion, but now Treyarch should be the ones producing COD - Infinity Ward should leave COD forever.
CoD4>CoD:WaW>CoD:MW2 for me. Havn't played the older ones in ages (meh, I do remember CoD2 being quite good for its time).
Infinity ward made an awesome game in CoD4. Treyarch followed up fairly well with WaW, just the campaign sucked balls compared to the CoD4 campaign (then again, CoD4 campaign was a pretty damn awesome campaign), CoD4 multiplayer was also a bit better, but WaW does have zombies. Then MW2 just took all the nooby aspects of CoD4 and made them x9000 and took away all good aspects.
Also, they took away dedicated servers, which every PC FPS has, unforgivable.
Overall: Infinity Ward>Treyarch, they made CoD, CoD2, CoD4, all great games compared to a decent CoD expansion, shitty CoD3, and good WaW
Recently: Treyarch>Infinity Ward, they didn't decide to remove dedis and didn't go overboard with perks, attachments, achievements, etc.
