I am trying to buy the most reliable case fan known to man. The Fans noise level can be as loud as it wants. The fan has to have good enough airflow. The fan has to last a long time. Those are really the only requirements. As far as size.. probably the most common sizes whatever that happens to be. Size is not really important to me. But if I was forced to pick a size, I would have to pick the most common as of right now. Which seems to be: 120mm, 140mm, and 80mm seem to be the most common types as of right now.
A good example, and I don't want people to dwell on this very much, is enterprise. What type of fans does enterprise use? I know IBM sometimes uses a company called Delta.