Over time, I have tried pretty much all of the major brands of processed mozzarella cheeses but unfortunately where I live I have not been able to get a steady enough supply of any of the better brands to be able to conduct multiple experiments in my oven with any one brand. However, of all of the brands I have tried, my favorite is the Grande mozzarella cheeses, and especially the whole-milk version. Even though some people prefer the flavor of other mozzarella cheeses, in my oven the Grande holds up the best for just about any style of pizza I make and for pretty much any bake temperature and bake time I use. With lesser cheeses, I have to worry whether they will break down and become oily or brown prematurely before the rest of the pizza is done. In fact, when I use such cheeses, I often reverse the sauce/cheese sequence and put the cheese down first and then the sauce, which I usually put down in dollops to shield a good part of the cheese from direct oven heat during baking but still leave some of it exposed and visible after baking. Sometimes I put the cheese down in the form of slices, which has become my standard method when making cracker-style pizzas that require a fairly long bake, even after using a pre-bake of the crust. An additional positive attribute of the Grande mozzarella cheese is that it freezes better than most other brands and is not as crumbly when shredding after defrosting it. In my case, I cut the Grande mozzarella cheese into fractions of a pound and seal the pieces in my Deni vacuum sealing unit before the pieces go into my freezer.
Grande cheeses are available from a few online sources or by telephone, but the shipping charges makes the cheeses very expensive on a per-pound basis, and even more so if you are some distance from such suppliers. Some wholesalers will sell to individuals but the minimum purchase requirements are hard to meet as individuals. Some Whole Foods stores carry the Grande mozzarella cheese under their own name, but not all Whole Foods stores do that. The ones I have heard of that do that have been on the West coast. In my case, I was unable to convince the Dallas Whole Foods store to order the Grande for me. I usually wait until I am back east where I am able to buy several pounds from a pizzeria, albeit at a stiff price. But, for me, it is well worth it. When I run out of the Grande, I try to find cheeses locally that are produced by Saputo, which has several brands of pretty good overall quality. Some of the Saputo brands that I have tried are Frigo (noted above by petef), Stella, and Dragone. I have also had good results using the Precious brand of mozzarella cheese as noted by scott, and also with Polly-O. However, with all of the brands other than Grande, I take into account how I will use such cheeses based on the type of pizzas I plan to make and how I plan to bake them.
Another factor to consider when it comes to the “runniness” of mozzarella cheese is the shape and size of the shred. I believe it was scott r who said in another thread that commercial places have a longer shred—either as purchased or as shredded using attachments to their Hobarts—than what we can produce at home using a standard box grater (or maybe a food processor). That different shred may be responsible for the stringy and stretchy character of the cheese on the pizza, possibly along with using whole-milk mozzarella cheese, as scott noted earlier.