You know, I have tried several different rubs for pork, chicken beef etc. Most often I like a simple rub of equal parts salt, pepper, garlic powder and a dash of crushed red pepper. I tried for the first time coating my ribs in yellow mustard prior to applying the rub and I really liked it's adhesive quality and little extra tang, I will be using that method again.
All that being said I really, really, really liked how the rack of ribs turned out that I applied Grill Mates Cowboy Rub to. For now, this will be my "goto" rub. It produced a thick, dark bark on my ribs and I can't wait to try it out on a pork butt or something. Check out the link below to the exact rub I am talking about.
http://www.mccormick.com/Grill-Mates/Flavors/Dry-Rubs/Grill-Mates-Cowboy-Rub