Thats a tough question, as there are so many really good Italian builders out there. I'm sure there will be a lot of disagreement, but I'll give it a shot. I figure by the time this thread reaches 10 pages, every Italian builder will be mentioned!
Ernesto Colnago - While Ernesto didn't really do any building on his own, he revolutionized the industry through his unique geometry and salesmanship. A few here will not like this pick, but his importance in the industry is undeniable.
Sante Polgiaghi - he was considered a true master builder and his bikes are still highly sought after.
Irio Tommasini - simply beautiful bikes
Ugo DeRosa - again, very tasteful bikes
Basso Brothers - not normally considered at the top, but I think they are some of the best riders out there so I have to include them and they are still very much in business these days as well.