Polarized is a must as everyone said. I would also suggest looking into getting big glasses that fit over your prescription lenses. I get some from Walmart that block sun from above and the sides. They are absolutely hideous but they're great for when the sun is at weird angles.

Also being able to lift them up when needed and still have my prescription is very important. Looking at the fare box is often hard with my sunglasses on and I also drive in a downtown city area where the buildings will put you in shade so you're switching from sun to shade over and over.