My favorite was CCA without a doubt and the beach area was one of the most stunning areas ever created for it's time. Seabed was the shit too because you honestly felt you were going in deeper when you go from this serene aquarium looking place to an almost sewer-like area that suffered damage from some bio-weapon outbreak which is what essentially happened.
Also if that long hallway with the lone Delbiter and two traps didn't make you sigh everytime you got there, you're lying. (Unless you were me on Whitill trying to get a Rika's Claw which I was able to get after grinding it for a week straight.)