I know others (such as Olm) have posted about this place already, but I have to say that 15 ("fifteen") has the best pizza in all of Taipei, imho.
The Margherita at 15 is quite simply excellent. Excellent crust, high-quality cheese, and a very good sauce. They have an imported oven from Italy, I think, and it does a great job. For me, this is the true test of a pizza place--how well they do the Margherita, the most basic of pizzas--and 15 certainly does this one well. For me, it has become my old stand-by.
Add on very friendly staff and a convenient location (for me) near Taida (and also near the corner of Fuxing and Heping), and 15 is, without a doubt, my favorite pizza place in the city.