Ha! You ask a funny question along the lines of the Ultimate Question--Life, the Universe and Everything. Since all of us Hitchhiker's around the Galaxy know the answer is 42, that is my answer here.
Where can you find the absolute best? In about 42 places--at least that's the number of responses you will get from 5 Chowhounds.
I would suggest you peruse this board as there are hundreds of responses to dozens of posts.
If you live in Chicago, then this is a question that requires a quest of its own.
Start with Lou Malnati's, Giordano's, Pizzeria Uno, Pizzeria Due, and Gino's East (not what it used to be, but had to throw it in for the 'tourist trao' perspective), and then work your down the list of honorable mentions in various CH posts/threads from the past.
. My personal fav when I lived in Chicago was Giordano's - consistently good, nice ratio of cheese to sauce to toppings, and the crust was flaky with a slight cracker-like texture. If you're new to stuffed pizza, you have to try it with spinach.
If you're just visiting, pick any from the list and you'll probably enjoy it.