We took the Free City Center Tour and the Teotihuacan Tour (30 USD per person). The free tour was good, but honestly a bit long. A LOT of standing in place and not a ton of, let's say, narrative coherence to some portions of it. But, got into some places in the city center you can't get into otherwise and got good background on some of Mexico City's best-known sites. Went 2+ hours though -- we left the tour early and I'm not sure how much longer they went.
Teotihuacan tour was great. The reliance on public transportation wasn't the best, but then, you get what you pay for. For a 6+ hour trip, the price can't be beat and we climbed the Moon and Sun pyramids, and got lots of good information. Take a tour for no other reason than that we would not have been able to find the bus stop going back to Mexico City on our own.More