Gotta say the Intense Micro knobbys. I got a set last week, and go from masonite half pipes to dirt and stick great on both. Cant say that for my friends' table tops or k-rads.
I have DMR Moto RT's and they have loads of grip at the skatepark and as long as i stick to reasonably hard dirt they are great on dirt as well, they wont engage at all on stodgy mud though.