I use the Bayou Classic SQ14 which is a 55,000 BTU burner and I was able to bring 10 gallons of wort to boil in about 30-45 minutes yesterday.
Here is the burner on amazon.com, it is pretty nice and doesn't consume too much propane.
http://www.amazon.com/dp/B0009JXYQ4/?tag=skimlinks_replacement-20