If you've got the time head to A, especially if you want quiet and space (head to the ends). B also great though as long as you hang out in the Champagne Bar. Also B has a smoking section.

C is fine for shirt stays and you can pour your own Heineken from a draught tap which is nice.

For showers also A is best. There's supposedly a new lounge (J/F combined) at the end of concourse C (lower C gates) but I haven't been there.

A also has a great and spacious business center if you want to do some work.
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Contrary to OzTennis' view I often take the train to A just to use the lounge, sometimes even when departing from C (these days I just hit the Champagne Bar in B though).

Also there's only one lounge in B, OzTennis when you speak of B16 I think you just mean the other entrance to the J lounge from the smoking section?

EDIT: also if early morning and in need of coffee the B lounge has complimentary Costa coffee which is better than the machine coffee. And if you want ice cream ask for it, they have Haagen Dasz but not always on display. Doughnuts are also delicious