Rib Rub?

It depends on the heat level you want.  Cayenne will definitely add a bigger kick and there are many other chile options out there.  I like using a combination of ancho (think mildly hot raisin, good fruit/earthy flavor with mild heat) and cayenne/guajillo/chipotle powder. 

You can calso add chili powder which is a combination of chile's, cumin & usually some oregano, garlic powder and salt.  This will add some heat as well as additional flavors to your rub.  You also always have the option of doing both.  That is the great thing about making your own rubs, you can tinker with the ingredients and ratios and come up with something that you really love. 

Once you find a recipe that you really like and are happy with I would find a good spice supplier (I order mine here http://www.wholespice.com/, but there are also many other online options and you may have some local brick and mortar ones) to order the spices for your rubs since they tend to move inventory quickly so you can get really good quality, fresh spices for your rubs.

Chipotle powder

Here's what I've been using, which is from America's Test Kitchen:

wet rub:
6 TBSP yellow mustard
2 TBSP ketchup
3 medium cloves garlic minced or pressed
Combine above in small bowl, then spread a thin layer on the meat.

dry rub:
2 tsp ground black pepper
1 TBSP paprika
1 TBSP chili powder
1/2 tsp cayenne pepper
1.5 TBSP kosher salt
3 TBSP brown sugar

Combine
dry rub ingredients in a separate bowl, then sprinkle over both sides
of the meat, then wrap meat in plastic (I usually use stretch-tite food
wrap, and then if it'll fit, throw the wrapped up meat into a 2G zip
lock, to avoid any leakage).  Refrigerate 8-24 hours...generally
overnight.

Last ribs I made (3 full slabes of St. Luis cut spares), I used this rub combination, but I made 1.5x the recipe, and it was just enough.  I have no love for spicy foods, and my wife isn't a huge fan of fresh ground black pepper, so I reduced the ground pepper 50% and the cayenne pepper by 2/3.