What is the best place in Mexico to visit? I'd like to see ruins and nice locals.?

Look at the map in the link below. Each of the pyramids is a different Maya ruin on the Yucatan peninsula. The peninsula is divided into three Mexican states: Campeche, Yucatan and Quintana Roo.
Probably the three best ruins to see are Chichen Itza, Uxmal and Coba. But if you fly into Cancun, you can rent a car and drive to most all the ruins on the map in a couple of weeks.

I wouldn't just stay in a single place. The ruins are scattered around and if you move from city to city you'll see more. For example, Mexico City has ruins, Oaxaca, the Yucatan Peninsula has several scattered. So if I were you I would start in Mexico City, Puebla, Veracruz, Oaxaca, and so on. Traveling in Mexico is not expensive and you can stay at hostels very cheap too.

Malinalco
Chichen-Itz谩 (highly recommend)
Tulum
Monte Alb谩n
Calakmul and Edzn谩
Palenque
Xochicalco
Teotihuacan

The Yucatan is fantastic. Shangri-la Carribe Resort is on the beach, walking distance to Playa del carmen....halfway between cancun and the ruins of Tulum. A short boat ride to Cozamel,. Minutes from Xelha (national marine park) and Xcaret (underwater river, dolphins etc) Wonderful scuba diving and snorkling) Enjoy

Oaxaca has fascinating ruins, plus many surrounding villages each specializing in a particular art work, such as pottery, rugs, weaving, wood carving, etc. The Zocolo is beautiful, and pedestrian friendly, the food outstanding, and don't forget the chocolate! <G>