I think simple rubs are best. Too many flavors overwhelm the meat. I use this rub for nearly everything:
1 part paprika
1 part sugar (brown or turbinado)
1/2 part kosher salt
1/2 part fresh ground pepper
The flavor from this is amazing considering only four spices are used.
This goes good on everything. If you want other flavors, you can add onion powder, garlic powder, oregano, cumin, rosemary thyme, cayenne, red pepper, green chile; at 1/8 to 1/4 part or less. The more spices you use, the less of each you should use. But starting with the basic rub, you can make a lot of different flavors depending on your taste. You can also vary the basic recipe if you find it too salty or too peppery or too sweet. Or not enough. Just increase or decrease the quantities of what you like or don't like. Cook, taste, modify. Repeat. Repeat. Experience is the best source.
Having said all that, a brisket smoked with just salt and pepper at 50/50 proportions is wonderful. The Texans call it Dalmation.
There are certain flavors associated with certain foods, chicken with parsley, sage, rosemary,and thyme for example, so experiment with these flavors when you smoke those meats.
Flavors go quickly from spices and herbs, and hence rubs, as they age, so this is one advantage of making your own from fresh spices and herbs. Try the Spice House and Penzey's for fresh herbs and spices. If you have leftover rub, store it in the freezer.
The point is to start with something simple, taste and discover what you like about it and don't like about it, modify to reflect your tastes. It sounds daunting, but it isn't. Your taste buds know more about what you like than those who write recipes and frequent this list. Including me. Keep notes. Tell us how it works out.