Really depends on what you like as far as taste and texture, Norm.
For traditional pies, I used to use whole milk mozzarella, but it didn't have the "pull" I like that I get from part skim Mozz, so that's what I've been using for the past few years. I've noticed the taste and texture profile varies considerably with different mozzarella brands, as with any cheese. Turns out the house brand they sell at the local Smart & Final stores has both the best taste and texture of many part skim mozzarellas we've tried, and it's way cheaper. Go figure...
Sometimes for traditional pies, I'll add a bit of provolone, otherwise, I try to keep it simple.
For authentic Neapolitan pies, I'll use fresh(Buffalo) mozzarella.
