I'm not too interested in how few ingredients I need--mostly because I have access to so many--but I don't think lots of ingredients are necessarily necessary.
If I was required to skimp--
--then I'd start with Paul's version but it would be salt, green pepper, garlic and onion.
I'm not big on all that much sugar and use paprika only as a carrying agent; not often. I do like other ground chilies in rubs because they work so well but don't always use them--it depends on the direction I'm heading with sauces and sides.
I infrequently make rubs that don't include at least a couple herbs and usually 2-3 additional spices. I'm looking to highlight, boost, contrast and complement the flavors developed in and on the meats during cooking. These include the obvious cooked meat flavors but also the dozens of flavors created through caramelization on the meats surface (of the meat and the rub), and the flavors imparted by the smoke if I am using smokewood.
My biggest gripe with commercial Q places--and, for that matter, with most steak houses and numerous restaurants when it comes to the prep and serving of many grilled meats--is that the results are often so boring. Two bites and I'm ready for something else. I do not feel that a properly blended rub properly applied (this is critical) masks the flavors of the meat and dispute the notion--it's my opinion of course--that salt-and-pepper-only or minimalist rubs are better. They might be if the meat is meant to be sauced a bit, or there are other complementary flavors going on with the sides, but I do not care much for barbecue that
must be sauced and I find the simple-to-no-rub approach horribly lacking. (This is common in many places but most notably central Texas. The meats are often perfectly tender but the flavor is flat and one dimensional to me. Most are salt-and-pepper-only places.) I love sauces and love to make them but, in barbecue, I like the meats to stand on their own first (and to me this requires a good rub) and then the sauce(s) to work with the results on the plate. In non-barbecue--say a dinner of a grilled steak, a roasted lamb loin, or a duck breast grilled to med-rare--then my rubs tend to either be less invloved or more lightly applied, or both, as I'll be focusing more on the sauce and sides as the flavor complements but this isn't always the case.
I know I might be out on a limb here and that many might disagree but, frankly, I have never had anyone tell me that they would have preferred the grass-fed filet I just served them (or the bison rib-eye, or the spare ribs, or the chuck, duck or brisket) to have only salt and pepper. Depending on what I am cooking and what I want the results to be on the plate I might make a rub more involved or less, I might apply more rub or less, but I'm always going to rub!
Enough of my rambling. I am not of the one-rub-fits-all mindset and prefer to make specific rub blends specific to the meat I am cooking and the result I want to achieve (and apply the rubs accordingly), but were I to be on a desert island with a cooker, fuel and all the meats, but only allowed the least in the way of rub ingredients then the following would be my hope: salt, pepper (probably green), garlic, onion, thyme, marjoram, aleppo or hot New Mex chile, coriander, clove, allpice, cumin, ginger and bay. I would hope that the island god would alow me to mix in the amounts I prefer, perhaps eliminating one or another depending on the meat I was cooking but if it didn't--or if I had to cut the list dramatically--I'd likely pull a Geraldine Page in Woody Allen's 'Interiors' and walk out into the ocean and drown myself.