Preparing Meats using Dry Rubs
Premade blends

If you're trying to buy something premade, look for blends without filler ingredients. For example, if you see sugar or salt as the first ingredient, that's a bad sign. Sometimes you'll also see large amounts of garlic or onion powder. Yes, you may often want salt and even sugar in your rubs, but you'll probably be happier adding them yourselves than paying a dollar an ounce for them.

It's also a great sign if they list all the ingredients, not just "spices".

And of course, you want it to not be old, since ground spices lose potency over time, but that's hard to tell without opening it. You may be able to get a sense for decent brands, or know that your store replenishes stock frequently so it hasn't sat on the shelf forever.

Beyond that, it's mostly up to your preferences. You'll just have to learn what spices you tend to like. It can be the highest quality in the world but if you don't like the spices, you won't like the rub. Keep in mind that different things do taste better with different meats; if you want a starting place, look for recipes with that meat, and then look for premade rubs with similar ingredients.

If you can't find anything you like in your own grocery stores, but still want to look for premade, don't give up. Just start looking toward smaller producers. Sure, they may not be in your grocery store, but there may be some great local places, and a lot of them have online presence now so you can use other people's local places too!

Just to give you the spirit of the idea... Your profile says you're in Chicago - I've heard of The Spice House there (since they also sell online), and they do seem to have some rubs. I don't know if enough good barbecue has made it up there, but if you can find any barbecue restaurants that sell rubs, that could be a good bet too. (I ate at this place last time I was back home in Texas and they even sell some seasonings online. Haven't tried them, but the barbecue was certainly good!)

Making your own

This is really an incredibly broad question, as you can see from the breadth of recipes you'll find if you search for recipes for rubs. There are really no "base ingredients" that make a rub good. As with premade things, your spice preferences will be personal, and depend on what you're using it on too.

But all that is really good news. You can find tons and tons of recipes online; trying a few and seeing what kinds of things you like will get you pretty far. In particular, if you grind your own spices, basically everything will be awesome, and your favorites will be down to personal preference. So we can't really tell you which ones you'll like best, but you can start to find out, and if you're grinding up single batches, you won't be wasting much along the way.