Would definitely recommend RIAT even though I've only been once. There are spectator packs available that'll get you in on the rehearsal days as well as the show days. The ideal package would be at least 4 days (arrival, show, departure). I for one certainly intend to go again.
Waddington and Duxford are two shows that I would like to see, Duxford especially for the WWII stuff.
Don't know much about Cosford.
At the end of the day it depends on what you want to see, modern, pre-modern, or vintage?