If you like real pizza you have a big problem in Sofia.

If there is only 1 place I should recommend it would be Spaghetti Company in TZUM.

The other 3 places that offer decent pizza would be Pastarito in the CCS mall, La Piccola Casa on Boris III (one tram stop from Pirogov) and, as Elenko said, the Osteria.

And pray there will be an Italian festival again next summer - they had amazing pizza this year.

Do avoid places like O!Shipkam, Ugo, Victoria or any other pizzeria chains...