My two cents:
For brisket, oak is THE wood, and if you don't have that, hickory needs to be in the mix. I usually mix hickory with apple and maybe some pecan, but I'm gonna get some oak before my next one as I'm convinced it's the best wood for beef. I've used just pecan for brisket before in the uds and didn't think it quite measured up by itself.
For pork shoulder, hickory is THE wood, but it's good mixed with other milder woods like white oak, pecan, and fruitwoods.
Chicken and ribs are fine with any of the above as long as the smoke is particularly clean and light, but the robust flavors of the nutwoods aren't necessary. I often just use just a fruitwood, but pecan should do just fine.
Hope this helps,
Dave