If you look for an optician only (buying frame and lenses), I can recommend you Europtica at Av. Pueyrredon 1997, close to Av. Las Herras. I compared a lot and my first decision was optica blu in recoleta (Arenales1500), which was really bad (took 4 weeks, scratched glasses, so money back ). At Europtica they speak English (which not so many do) and the service and also the work was perfect. You should compare the prices, they can be very different. Europtica is not the cheapest but the prices are okay.
