I have done a lot of snorkeling, but am a diver now. Only ever been to the Caribbean. It is funny how that changes your perspective, but...
Best Turtle Snorkeling, For snorkeling all day long with huge turtles, Akumal, Mexico - the reef is not in good shape but the turtles are always there, as well as tons of rays, barracuda, fish balls and other fish species. This is all to be had right off of a sandy beach. What a great place.
Best Shore Snorkeling, St. John is also a really good snorkeling destination. Especially if you are there for a week and have a jeep, there are many bays and hikes to beaches with just amazing snorkeling, this is from just last year that I visited. I saw fish and species there that I had only seen diving before. If I could not dive anymore that is where I would vaca in the Caribbean. It is really nice to be on your own schedule and just drive/hike to good spots. I was only there a week and found 3 or 4 incredibly good snorkeling spots. Once again the reef is not in the best shape, but better than some places in the Caribbean. Topside not nearly as awesome as Mexico. I would put Roatan as second in this category, as there are several really good shore snorkels there, in fact I have never been snorkeling on a boat there and was content.
Best Boat Snorkeling - Caye Caulker, Belize - the healthiest coral I have ever seen snorkeling and better than on some of the dives I have seen. Vibrant colors and variety. Really beautiful. Have to take a boat every time, though. I would also throw Klein Bonaire in to this category as a close second.
Here is where I have been diving/snorkeling, for context:
Grand Cayman once, Roatan twice, Antigua once, Cozumel three times, Akumal once, Belize once, Bonaire once (diving only but paid attn to snorkeling), St. John once (diving and snorkeling)
I know this is a Caribbean list, but I hear French Polynesia is incredible snorkeling and if I hadn't gotten dive certified I'd be headed there next.