What kind of passport do I need to enter to Italy?

I am a British and Algerian Citizen, I traveled from london Heathrow to Algeria now I am in Algeria, when i go back i will travel to Rome via Fiumicino airport on Alitalia Airways for a couple of days. so when i get to the passport control at Fiumicino Airport what passport i have to show them the British one or the Algerian one?