Since fewer people in Mexico have cars, the buses are much more heavily used. This means that many buses are available to most destinations, from third class to luxury. There are several classes of buses in Mexico and many bus lines. Remember that not all bus lines are equal, and first class on one line may not be identical to first class on a different line. A quick look at the buses should help you figure out which line has the newer, better luxury buses, which buses look like Greyhound, and which line is running old school buses.
Some bus lines offer more than one class of service.
After careful study, I have concluded that the company's own ratings may not match my definitions, so ask local advice or look at the vehicles with your own eyes. ADO, Omnibuses de Mexico, and Primera Plus are some of the major first class bus lines. In most cases, more than one line operates on the same route, so if you don't like the schedule, try another line. Most buses in Mexico are run by large companies, rather than by individuals or by the government. You generally pay when you get on, usually a peso or two.