Most of the margaritas I have had in Cancun are lousy. They say that they use fresh lime juice - but most of the time it tastes like it came from a mix.
On a personal note, I much prefer the taste of cointreau or Grand marnier to the more typical Controy you will find they use in Mexic.
We have had good margaritas at La Destileria (I'm a traditionalist - I never get the fruity ones - just the regular lime). I also had a great one at Fiesta Americana Coral Beach at the La Jolla restaurant. It was their premium margarita and was small but delicious.