Some people who go with a tour group are disappointed to find that they're herded around to various souvenir shops, and don't spend as much time at Teotihuacan as they'd like. But not all tour groups are bad! Just do your research and be picky about which tour provider you choose. Viator offers a wide selection of highly rated day trips to Teotihuacan that fit a range of budgets. I'm also a big fan of Tour Radar, because you can see tours from multiple providers and read reviews from travelers who've taken those exact tours.