There are hundreds of great sauces you can make and experiment with. I am partial to Steven Raichlen's Dr Pepper sauce. I know...some barbecue purists scoff at Raichlen, but damn, I can't turn down a recipe with DP. Along the same lines, some of the best rubs out there that I've found are the dizzy pig rubs. The Link Again, you could make your own, but that's a bigger pain than making a little sauce.
You can't buy it in the grocery store, but i remember eating at Luling City Market in Houston a few years ago and the sauce was AMAZING. You can buy it there but if you're in Austin....
I like to mix the Lauren's Habanero sauce from Salt Lick with the regular Salt Lick BBQ sauce. I like it a bit hot, but not as hot as the Habanero version alone
I guess it would have to be something from Salt Lick cause that's what they're famous for, SAUCE.
My favorite sauce would be whtever The Pit in Georgetown uses on their chopped beef sandwiches to kind of moisten up the burnt ends and such that they use to make their chopped beef, nothing else needs BBQ sauce.