I like all the Call of Duty games except 3 and World at War, which were not developed by Infinity Ward. They're consolified CoD, and the weapons are a lot less realistic than in the other titles.
My favorite installment was developed by a great developer, Gray Matter. The United Offensive expansion pack for Call of Duty 1 is defintely the best of the series, for the simple fact that it was released when the engine was already showing its age. However, this allowed the developer to have INSANE amounts of people on screen at once. There are battles in United Offensive where you can have upwards of 100 soldiers on screen plus tanks and aircraft. You won't find chaos even approaching that in any other CoD game. The battles also seemed to rely less on the heavy scripting you see in the other games.