CoD 4, easily. Back then things were simple and thus balanced. You get 1 UAV, 1 Airstrike, and 1 Chopper, back then getting kills was much more reliant on actual FPS skill rather than seeing who can get the most chit in the air to kill the other team. WaW was decent, but the tanks ruined it. Rarely would both teams use their tanks, and for water reason the team that isn't using tanks ignores the tanks all together and does nothing to stop them. This team is destined for failure...and the team that I seemed to be most likely to join when joining a random game. >.>
As for MW2, BOPs, and MW3, that's when they just completely jumped the shark with the killstreaks. Don't get me wrong, I thought it was pretty sweet to have a choice of killstreaks as well, but the novelty gets really old really fast. MW2 got to the point where you had to choose between using Cold Blooded so you don't get murdered by some form of Deus Ex Machina from the sky or Stopping Power so you could actually fucking kill someone. Well BOPs simplified things by getting rid of the stopping power perk. Ok, well now it became a choice of using the camo perk orrrr just straight up dying. And now there's MW3 in which if some kind of killstreak isn't wiping out half of your retard-squad every 4-7 seconds you must be in a room by yourself. Note that the above descriptions are based on generalities.