The best rub in the world is the rub you like best. More often than not the rub you like best is the one you make yourself. So where do you start? Generally dry rubs start out with a few basic ingredients. Remember that a rub is not only a mixture of spices and herbs, it is a mixture of flavors. A good rub will have a balanced flavor that adds to meats, but doesn't over power them.
Amount: First of all you want to decide how much to make. Dry rubs can be stored for months, but after time they begin to loose their flavor, so if you won't use it all in a few months then you've made too much. When you are first starting out with a rub make it in small batches, but be sure to record how you made it. You don't want to come up with the perfect rub only to forget how you made it. I save old spice bottles to hold my rub and put a label on the side with the recipe. I also record the date I made it. This way I know exactly what I am using and how long it's been sitting around.
Basic Flavors: You want to start your rub with the basic flavors: salt, sweet, sour, and bitter. Okay, you want to start with salt and sweet, specifically sugar and, well, salt. If you choose plain salt and plain sugar you will get a very basic base with which to start. Of course most people who have been making rubs for a while use something different. Sugar can be white, brown, turbinado or any other kind of sugar you like. If you want to make a wet rub then you can use molasses. These add a different flavor to your rub that can really enhance the taste. As for salt you can go with regular table salt, sea salt, kosher salt, or you can use a flavored salt like garlic, onion or seasoning salts. Salt should actually be measured by weight not volume but as long as you understand the differences between types of salt you will be okay (see: Salt). Simply combining something like brown sugar and garlic salt will give you a pretty decent rub. Start out with a cup of each and you are ready to build up your rub.
Building Flavor: These two ingredients make the base of the rub. From here you build on the flavor and the color that will make the rub something that adds to the meat you use it on. For the sake of argument let's assume that you are using one cup each of sugar and salt. Now add 1/2 cup of paprika. Paprika is great for color but also for its mild flavor. The color is strong so you don't need as much paprika as you do with the other two. Now you have something that really looks like your typical spice rub.
Adding Heat: From here you can start building on the flavor of the rub you want to make. In truth you can simply add some herbs and call it done. A 1/2 cup of oregano or Italian seasoning will round out the flavors and give you a good rub. However it you want a rub with some heat you need to think about adding pepper. Depending on the heat of the pepper you add you can go with anything from about 1/2 cup to a teaspoon or two. If you are adding cayenne I would suggest about 1 tablespoon for a fairly hot rub to 1 teaspoon for something with a touch of heat. You can also choose a variety of chili powders, crushed red pepper flakes or any other dried and ground pepper.
Herbs and Spices: To finish off your rub try adding some herbs. As I said before some oregano is a great addition. Remember that it is easier to add something to a rub than it is to take it out. Of course you can simply add additional base ingredients if you end up with something too powerful, but I find it better to be conservative with the herbs and build up later. The best tool for rub building is a good collection of spices. Line them up and add them as you see fit. If you start out making small batches of a rub you won't end up with a large supply of something that just isn't working. Experiment and you will soon have a mixture that truly suits your tastes.
Once you start getting some rubs together, try them out on different meats. See what you like and what you don't. In no time you will have a collection of rubs for every meat and for every occasion.