The gameplay in COD 4 was perfect, it wasn't too fast paced and involved tactical thinking, but not too much that it gets frustrating and tedious.
World at war was also great, basically the same as COD 4 but with a world war 2 theme and zombies,
Modern Warfare 2 is by far the worst of them, the only thing I loved about it was playing special ops, the multiplayer was very weak and too chaotic for my liking. Black Ops was also similiar to this but slightly better and had the great zombie multiplayer addition which I love!
Call of Duty 4: Modern Warfare is my favourite FPS game of all time. If they combine the greatness of COD 4's multiplayer gameplay with new maps (including zombies), weapons, features, gadgets and game types then Modern Warfare 3 will be a big hit in my opinion and I'll buy it, let's just hope that actually happens!