Like JCavilia wrote, the biggest tires that will fit. I've been riding Compass road tires and have had good luck on gravel. But the bike is 650b x 42mm and most 700c bikes won't fit a tire that large, but if 32 or 38mm tires would fit your bike, that'd be good.
I just read a blog post about traction on gravel and the use of road tires. The gist of it was that the loss of traction wasn't from the interface of the tire on the gravel, but from the gravel on gravel interface. The point was that the "buoyancy" of a larger diameter tire keeps it on top of the gravel rather than riding thru it, and it's the gravel layers under the tire that are slipping.
In my experience this holds true. Riding gravel, on my tires, is not an issue until the gravel gets deep and loose enough to overpower my tires. The larger the tire, the more float it has to ride on top of the gravel as the gravel gets deeper.