If you could only carry one fly box on a trip, what flies would you pack?

Sort of a newbie question coming from a seasoned angler, but the reason I ask is that often I find myself with spare time during the day, and rather than take everything down to the water, I would just grab my rod from behind the seat of the truck, a pair of forecepts, and a small to medium fly box from my guide bag.

On my last trip I was thinking I should make one box as my go to box, a small to medium size box that will fit in with my rod and reel in it's case, along with a pair of forecepts because often I end up hopping a ride with fellow anglers or a spur of the moment "take a couple casts to see what's out there" locations occur frequently.

So if you had one fly box, what would you pack in it?