Which one is overall the best? Primary considerations are selection, price, and reliability. Which stores should you avoid?
I prefer Costco and Walmart for price, with Costco being the most helpful and friendly. Farmacia Paris on Yaxchilan often has things that the the other two don't have in stock or don't carry. If you tell them you are Cancunense the price gets cheaper. If you have the Inapam card (you may not qualify, as it is like an AARP) both Walmart & Paris accept them for a worthwhile discount. Those are my go to places and unfortunately, I "go to" more than I care to at the moment, but whittling those visits down as fast as I can.
I find that everything I ever needed, Walmart was the most expensive. Now I only use Ahorra or Similares. Best prices for what I need.
I agree about Similares if generics are accepable and available. Ahorro is convenient to where I live but they never seeme to have what I needed. I do have their frequent flyer card though and a few points on it.