Everyone has their favorite, but for me PR has it all, I can pick and choose what kind of Island vacation I want to have.

Of the islands I've been to in the Caribbean, Puerto Rico is the best IMO. The reason I state this is I want both relaxed small town, off-the-beaten path beaches/areas, but a week at the slow pace is too much for me and my wife (so we need an island that has a great, exciting nightlife, a great amount to do outdoors is a plus. I also seek the local culture. Puerto Rico meets all our needs:

It is easy to experience/take in the local culture in PR, you may not understand what they are saying, but it is easy to find places where the locals hang out, festivals, bars/restaurants and you are accepted. Again, not so easy to do on other islands, very rarely do you find locals spots.

Besides the great amount of beaches, PR also has a rainforest and the center of the island is a lush forest. Great for hiking/exploring.

San Juan is a GREAT food city (and IMO all of PR is) and has exciting nightlife.

The slower side of PR is just outside San Juan and all over the island, just drive out.
Fajardo and Ricon are very "slow" areas as are Guanica on the SW side of the island.

I almost forgot to mention, no customs to go through and PR uses the US $.