So I've been swapping around the past few call of dutys for a few days, and I have to say that when all is said and done, BO is by far the most playable.
Saying COD4 is the best is the easy way out. It's still pretty fun to play, and not really all that broken, but it's not as refined or interesting as later installments. Not nearly as much weaponry and equipment to play around with. The maps that are good are great, but there's fair share of maps that just make you groan when you have to play them (wet work). The major silliness is about 1/3 of the players are psuedo-quickscoping with the a cog. It doesn't break the game, but it seriously detracts.
WaW has held up surprisingly well. Unlike the IW games, nothing is really broken. The MP40 is a strong SMG, but it's not a total super weapon at all ranges, and it doesn't make the game silly. The constant dogs are annoying, but overall it's still a great game to play if you dig WW2.
MW2....now this was the shocker to me. I've always loved MW2. It has the best maps, bar none. The intraclass balance/variety of weapons is excellent. But the game is just completely broken, almost completely unplayable at this point. 1/2 or more of players are quickscoping with an intervention in any given game. Another 1/4 are OMA/DC noob tubing. More often than not youre dead before you can even react, the weapons are so powerful. Youre constantly bombarded by voice messages for boosting lobbies. The killstreak stacking is ridiculous. It seems like all the people that played the game for fun left, leaving only the most hardcore of hardcore. It's just not a fun game to play anymore.
Overall, despite it's bland maps and weapons, BO is by far the most finely tuned, and least annoying/broken game. Sniper rifles are sniper rifles, not shotguns. The famas is overpowered but not game breaking, and the Uzi is underpowered, but not useless. However uninteresting treyarchs games may be compared to IWs, they seem to be holding up better over time.
If there's one thing I hope for MW3, its that they get rid of killstreaks. Theyre really what end up spoiling every game in the end. There still should be UAVs, helicopters and all, but I don't think they should be doled out strictly on the basis of killstreaks. Something more like what homefront did, where a steady positive benefit to your team such as capping a flag gets you a bonus, not just how long you can stay camped in a corner without dying. In either case I hope the new IW isn't so arrogant not to learn from BO. I can see BO being fun and playable for many years to come. Unfortunately, MW1 and MW2 are pretty much dead.