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The Yucatan Peninsula (Riviera Maya - - Cancun down to Tulum) promoters brag about their beaches; but it depends what you mean by "best" (swimming, snorkeling, surfing, para-sailing, walking ankle deep in the warm water on fine grained sand holding hands under the moonlight as you lose yourself in each others' eyes,,,,,) and which side of Mexico you are budgeting for.

Cancún and the coastline of the Yucatán state of Quintana Roo are home to Mexico's most enticingly beautiful beaches, with powdery white sand and Caribbean waters the color of a Technicolor dream. Cancún also has the widest assortment of luxury beachfront hotels, plus more restaurants, nightlife, and activities than any other resort destination in Mexico. I visited these places a few months back via yatra.com and it was truly a great experience.