Primarily oak - not only does it generally have a great flavor with almost any meat, but it burns hot, consistent and long.
Fruit woods or other woods will depend on what I'm cooking.
Pecan pretty much goes with everything as well, so usually my mix is oak and pecan - for flavor and because it's readily available to boot.
I tend to shy away using mesquite on anything pork related. I may toss in a very small amount, but too much mesquite doesn't mesh well with most pork in my limited experience and opinion.
Apple and hickory go great with pork.
Mesquite goes great with beef, though as time goes on I use less and less because it's easy to oversmoke with mesquite. Mesquite is usually a great wood to use with seafood.
A typical burn will generally be 50% oak, 40% pecan and 10% or less of another fruit wood.