Congrats on the upgrade to the D7100! My everyday lens is a 50mm f1.4 AF-S. For DX equivalent I would go with a 35mm f1.8 AF-S. I prefer primes over zooms and consider a standard lens a great all-around lens. In some ways they are more versatile than zooms because you also have better capability in lower light, are able to isolate subjects from a more out-of-focus background, and for most of the moderate range zooms out there, you have similar coverage simply by moving back or forth a few feet (or meters).
Plus they are small and light-weight and often quicker in terms of AF capability than zooms. Anyway, that's my plug for a prime.