Dynamat is going to be the best quality but the most expensive.
Fatmat is going to be the runner-up. Its quality is not as great but the price is alot cheaper. I believe I paid around $100 for 100 square feet of the stuff.
Also for the poor mans alternative there is some stuff from home depot for very cheap "ghettomat". Not as great as the others but it is cheap.
Sound Damping is a royal PITA to work with, It makes your car smell of asphalt for a weeks. And it needs to be installed correctly to work right. But if installed correctly it is a great investment
Also note that these products are extremely heavy. http://www.protectowrap.com/index.php