My preference, in order: D, A, C, B.

D and A are the biggest.

Details:
D is generally brighter and has a great view of the airfield. However, it can get crowded at times. There is a great somewhat hidden room when facing the bar, turn to the left and back a little, and it's around a corner.
A has the best showers in the AA system and quite a few cozy corners you can usually get some peace in. Also a bigger club, but more dim due to being on the inside of the terminal.
C is very similar to A, but without the showers. I actually like C quite a bit as it can be pretty quiet and not overly bright. The C club (somewhat like the A club) has many divided sections compared to D, which is more of 1 big open room.
I've only been to B once - nothing bad to say about it, but it's a hike over there unless you are connecting through the B gates, and I don't recall anything special there.

Hope that helps. If you have 3 hours, you may want to try a few of them so that you know for future trips. That's what I did back when I first became a member. With Skylink, this is possible and spend about 20 mins in each club looking around getting a feel for the place.