Use any hot red (or green for green Harissa) pepper you like just be sure you know how hot it is. I liked using Fresno peppers they measure from 2,500 – 10,000 Scoville Heat. Scoville Heat is the scale to measure how spicy (hot) peppers are. To help you get an idea of how hot your peppers are, note that bell pepper measure zero, jalapeno measures from 3,500 - 8,000 and Habanero is 100,000–350,000 on the Scoville scale. I love staying in the neighborhood of the jalapeno which is actually the heat of the Tunisian baklouti pepper that is traditionally used. Here is a guide of peppers to help you choose.