It depends on your taste, and most folks look for excitement & familiarity therefore Cabo/Puerto Vallarta/Acapulco are a few destinations that people go for the beach and girls, Cancun (cost a bit more money & is nicer), or Veracruz (bigger city with more activies, such as Spanish dance). Tijuana is just a border town that dedicate to American tourists (sex, shop & alcohol). Oaxaca is a nice place to visit in the southern highland if you are looking for culture & pottery. Merida is a land lock city, however it still is a nice place to visit with loads ruins in the surounding area, and is a few hours from Cancun. Chetumal, is a less develop town that border with Belize, and IMHO is nicer than Cancun, because the water & coral reef is incredible (Still in the Yucatan like Cancun & Merida). Chiapas border with Guatemal, also is a nice place to visit if you don't mind some unfriendly natives. And, IMHO the pretiest Mexican city is Villahermosa, locate on a river with greenery & beautiful parks that are dotted with white cranes & flamingos.
Best is get the Lonely Planet guilde & make your decission base upon your reading. Also, post here & I'm sure there are a few backpackers at ATOT can help you if you have more questions.