If you mean normal, run-of-the-mill touring then the Ortlieb Back-Roller is my pick. It comes in pairs, and a 40-litre combined capacity is ample, especially if you bolster it with a bar bag. If you want more then the Back-Roller XL Plus is about as big as one can feasibly get, but unless you're riding a dedicated touring bike I'd advise against this as you can easily overload the maximum system weight of a bike or the rear wheel.