Depends on where you live. Letterman plays better on the metropolitan coasts, and Leno plays better in the heartland. The producers tailor their respective shows accordingly.
I like Letterman much better because he has a deep respect for the history of the late-night shows. A lot of the skits he does is in direct homage(ripoff if you're cynical) to the stuff Allen, Benny, Paar, and Carson did. He also originates a lot of stuff that actually lets the audience discover different kinds of humor, silly or sophisticated. Leno just mines the same old club jokes that's safe, and tried and true in middle America. The only reason he beats Letterman is that folks in those areas watch more TV thans folks in urban areas where there are more things to do. I'm not knocking any region, that's what the numbers tell you.
The best in my time, however, is Carson. 30 years on top of the heap and he left without ever looking back. That's how you're supposed to go out.