I carry two pairs in my flight bag. For bright sunny days or 'on-top', I wear Ray-Ban Olympian I Deluxe, which are a gold rimmed wrap-around (way better looking than the predator), that look so fast that I swear I get an extra 10 kts out of the machine.No need to look prudish in Ray-Bans!
My all around utility glasses for all other conditions, particularly in cloud are Serengetti Driver's with a gradient lens so that you can see the instruments below the coaming more easily while not sacrificing the light cutting properties above. They actually improve the acuity of approach lights in reduced visibility daylight approaches.If you're going for just one pair, these would be my recommendation.
Oakleys are great sunnies, but hugely overpriced for what they are (particularly the Pro M frames).They are also not nearly as durable (with a coated polycarbonate lens). Count on replacing the lens once every couple of years (that's if you don't wear them outside of work!) at $70 USD a pop.
CHEERS!