I'm of generally a similar mind to BDL overall in what sort of rub I'd do, and his marinate and slather steps are good choices too.
So here's a rundown of a sort of general workhorse rub I use as a baseline, then tilt particular ingredients to match an ethnic flavor or particular cooking method. I'm targeting beef here generally.
I tend to salt the meat separately from the rub because I'm on a sodium restriction. This also lets me use the rub more thickly or thinly depending on my flavor goals, and not have to worry about getting too salty. Alternatively, you can let the rub marinate longer and have less concern about the salt drawing too much moisture.
1 part garlic powder--I usually start with the garlic and use that amount as my baseline amount for a part. My philosophy of rubs tends to center around balancing against a particular strong flavor.
2 parts onion powder--This 1:2 ratio between garlic and onion is one I've found pleasing. Most other rubs tend to be 1:1 which is good, but I find less balanced to my tastes.
2 parts paprika -- Shift this up or down depending on the amount of color you're looking for. These first three ingredients often clump and is another reason to use a grinder for mixing and breaking down whole spices fresh.
1/2-1 part ground chile--For mild heat, use ground Mew Mexico or California ground chile; medium use something like Guajillo; hot cayenne, or those others BDL noted are good too. I'm not a fan of chili powder for most rubs because the cumin in chili powder tends to strike me as the wrong accent too often.
1/2 -1 part ground black pepper--The larger the cut of meat, the more I tend to add.
1/2-1 part herbs--For beef in the general sense rather than a particular ethnic flavor profile, 1/3 thyme, 1/3 marjoram, 1/3 ground bay leaf. Beef, I tend to hit with herbs more strongly than I do pork or chicken. For even less herbal situations, I'll use an "italian herb" mix with a light hand just to give it some background but without anything distinct. I mix my own generally.
1 part mushroom powder--you have to make your own. Buy some dried mushrooms, I tend to use the dried Chinese type as they're economical, but adding a little porcini is nice too. I like to use a cheap coffee grinder for this process and other spice grinding. A food processor is OK too, but makes a coarser product. But when I cook beef, I tend to add mushroom powder to the rub. Also handy to mix into ground beef. I don't use this generally with pork or chicken unless I'm pairing them up in the meal with mushrooms.
1/2-1 part citrus-like flavors This arises in part from my sodium restriction, but I've been impressed with what it can do in a rub. Some ideas to use singly and together:
Ground coriander has a mildly citrusy tone I like and often pick for a gentle hint of citrus and also brings some warm tones to the rub without heat.
Ground sumac is one I reach for often. Besides a lemony flavor, it adds lots of soluble dark red color. This added color can be good and bad so think about what you want from the juices, fond and so on as that might not be compatible with using sumac
dried citrus peel. When I use citrus juice, I often use some of the zest too. If not, I'll thinly pare the fruit peel before use and let it dry on a plate on top of my fridge at home. When dry, I add them to a baggie or bottle in my freezer door. Then add a piece to my rubs for grinding. Lemon and orange peel are common, but lime and grapefruit are handy too of course.
Lemonade powder, sugar sweetened only as the sweeteners used in sugar free varieties tend not to be stable in cooking. You have to use this ingredient carefully as sugars easily scorch in rubs during searing of a roast or on the grill.
One last note on the citrus flavors, I have some citrus spice combinations described in this thread
as well as a good Lemon Garlic Rosemary Salt
And bay leaf grinds poorly in my cheap coffee grinder. I find it best to break it up in my fingers quite a bit, then grind with some other whole spices in the rub like the pepper corns to keep them from floating on the air from the propeller
I've got a cheapo Mr. Coffee grinder that I use just for spices and blending rubs. I've come to favor a finely ground rub. I like the mouthfeel of the resulting surface better generally.