Will be there Jan 2019 and looking for everyone's favorite (maybe off the beaten path) restaurants?
Hopefully you'll get answers here. Also, for a very specific question like this, you'll also want to check with at least one other travel website (the name of which will not be mentioned here).
Maffei
Off the main drag near the San Domenico Hotel.
Most excellent seafood.
If you're in search for the best Arancini try "Da Christina." We were on a cooking tour and our chef guide took us there for an evening snack. It was perfect since we had a large lunch. It was the best Arancini we had in Sicily! It's an off the beaten path find.
Away from the madding crowd is Al Giardino, right across from the public garden, on via Bagnoli Croci. It is close to the hotel we stayed at so we ate there several times. Great food, friendly owner who speaks 5 languages, and his whole family seems to be involved in running the place.
Not necessarily off the beaten path, but Rosso Peperoncino just off Corso Umberto near the Porta Catania 'gate' is fantastic. We dined there when they had just opened in October 2010, but my cousin was there last month and said it was as good as ever.
Osteria Nero d’Avola, several lanes and stairs below Corso Umberto. Excellent.
If you have time, take a taxi up to Castelmola (15 minutes and €15) and go to Taverna dell Etna. Stunning views and amazing food!
Also up the stairs off of Corso Umberto, Nettuno, fabulous seafood dishes.
We enjoyed Pizzaria La Botte for its wodfire pizza and Trattoria II Baccanale for its grilled chicken. Its lasagna was ok. Both are on Via di Giovanni and reasonably priced.