Flying over the Teotihuacán valley since 2014, with pilots licensed by AFAC — Mexico's civil aviation authority — which is the single thing to check before booking a balloon here.
Flights go at sunrise, which is not a preference: the air is stable before the sun warms the valley floor, and by nine it is not. That means leaving Mexico City at around four in the morning, or sleeping nearer the site.
What you get for it is the Pyramid of the Sun and the Avenue of the Dead from above in low raking light, with the whole grid of the city legible in a way it is not from the ground. They also keep an office in Mexico City, on Avenida Nuevo León in Hipódromo.