Da Gigino in Sorrento. Went two times....un-fussy, good Italian food. Pizza's are more than decent and everything is reasonably priced. Located in the very centre of town, it's nice to sit outside if you get a chance. Expect a travelling band to wander by, entertain you for 10 minutes and ask for tips... ;) Enjoy a gelato and a stroll down the main street after dinner.
In Siracusa, I too had a wonderful meal at Osteria da Mariano
. And two wonderful lunches at La Cantinaccia
In Agrigento, I enjoyed a beautiful lunch after visiting the Valle dei Templi at Trattoria dei Templi, via Panoramico dei Templi (as it was on the main drag overlooking the "valley", I was a bit hesitant but it was recommended and it was lovely, food and service).