There are for sure some other good places but right now my preference goes to La Pizza, in the back of 3.3 in Sanlitun. Prices reasonable.
Another great Italian I discovered is Bottega, in front of 3.3 in the hidden alley of Sanlitun Street. Another class, also pizzas but rather special dishes. La Pizza is the “normal” pizza.

 

Sorry for The Tree, you come second as for now for pizza.
Both restaurant are usually packed, so better arrive early.

And taking a funny picture in the supermarket under Taikooli…