Living in the US, Cozumel is the most accessible and affordable diving location for me so that's where I usually go. The flights are short, the dive sites are close, price per tank is relatively low, and food and lodging can be a steal depending on your traveling style. I'd rather spend my time diving that flying to a remote dive location when I only have a week for vacation.
If I had a month to go anywhere, I'd spend it island hopping in the Philippines! That's definitely a bucket list diving destination for me!