English Language or Eng. Lit (Improves written and spoken english standards, and therefore improves overall communication skills).
Any science, political or geography - sciences for the obvious reason (they are quite hard) and the other two as they demonstrate a grasp of the wider world.
FYI, when i was at selection (With three crap A Levels D,E,E), i was told that not everyone in the RAF needs to be an academic. I passed all of the selection tests, and was told i could do anything in the RAF.
Airborne is right about mental maths - make sure that you can do the speed/distance/time calculations very quickly in your head, and when your working on the tests, dont forget common sense.
One of the tests that i sat involved getting some cargo from point A to C, as quickly as possible, using a car and a motorbike. I cant remember the scenario exactly, but i nearly proposed loading the bike up with 600kgs of freight, before i realised that i would get laughed at.