Key point - if you're buying packaged cheese stop. Get it sliced at the deli, packaged stuff has additives to jeep it from clumping and sticking together. Jack, American both good melters. Mozzerella doesn't melt into a liquid, it just becomes stretchy. I use sliced pepper jack and Land o Lakes white american sliced thin. I stack it and julienne, then toss it all into a croockpot or saucepan to melt, adding enough half and half or evaporated milk to reach the right consistency.