Which would you recommend

I had a wonderful experience at a branch of this school in Playa del Carmen, Mexico. I have not been to the branch in Oaxaca but can vouch for them as a school in general. You can find out about them here http://www.solexico.com/

Of the three cities you mentioned, I think Oaxaca and Guanajuato are both good places to study. In Oaxaca, check out Instituto de Communicacion y Cultura, Solexico, Instituto Cultural, Amigos del Sol, or Academia Vinigulaza. Oaxaca is probably the least expensive place to study Spanish in Mexico, but some of the schools fill up in the summer, especially Instituto Cultural. In Guanajuato, look into Escuela Mexicana, Academia Falcon and Instituto Miguel de Cervantes. Guanajuato is a beautiful colonial city with lots to see in the surrounding area. However, there are no major Pre-columbian ruins nearby. Oaxaca has the advantage of being close to some major ruins sites and interesting native villages. The city also has a colonial flavor. Puebla has only a couple of Spanish schools and it's closer to Mexico City than the other two, but that may be an advantage for you.

I suggest you do some detailed research on each place and each school you are considering.

Buena suerte

wow, thanks for the info mracine and travelrm. I have heard a lot of good things about the instituto cultural in oaxaca. Thanks again for the advice everyone.