Kind of a stupid and predictable answer, but I picked Tank. He's just so iconic now, even for those who have only played L4D once or twice know the Tank doesn't fuck around.
I kinda wished L4D2 introduced a new Tank-like boss SI, something that would force survivors to immediately change their game plan for the next few minutes and shit their pants with fear when the music starts up. Maybe some kind of omnipotent zombie king that doesn't have much more HP than regular SI, but can summon hordes of common infected at will, and the survivors have to seek out and kill him to stop the waves. He'd be sort of a wandering, spontaneous panic event. There would have to be limitations on his ability to run and hide, of course.
