Like sauces BBQ rubs are a bit controversial and there are literally thousands of combinations out there.
Even the enthusiasts and well established BBQ Joints that don't use sauce or serve it on the side always use some sort of rub, even if its just salt and pepper. From sweet to heat there is no doubt that this is the very basic process for grilling and smoking, regardless if brining, marinating or injection methods were used.
A great technique that also serves to help create a bark, and serves as an adhesive for the rub, it to slather the meat with yellow mustard. The vinegar in the mustard, being acidic, acts as a tenderizer. The flavor of the mustard won't come through but will bind the dry rub and help the spices permeate the meat.
So slather the meat in mustard, douse liberally with rub, wrap in plastic wrap and let sit for at least several hours in the refrigerator before firing up the grill or smoker.
Here's a simple basic rub:
1/4 cup salt (course Kosher or sea salt is best)
1/4 cup brown sugar
1/4 cup paprika (smoked paprika is best)
3 tablespoons ground black pepper (fresh course ground is best)
1 tablespoon garlic powder
1 teaspoon cumin
1/2 teaspoon cayenne pepper
You can also find rubs in grocery stores alongside BBQ sauces.