But sober words of caution are in order: First, one person's oh-wow is another person's ewwwwwww. There's just no accounting for different tastes. Second, no matter what the attraction, beach, or trail, you need to pick the right day to see it at its best. And, third (here's the country-song-sad part,) some places have become so popular that the experience is no longer sure to be "the best."
Let's stick to beginner snorkeling. Here are popular places on each island that have been killed by kindness; followed by suggestions to nearby alternatives that are relatively free of crowds and high on scenic value.
Popular: Hanauma Bay. The bay (pictured) can be a zoo, featurng a full-parking lot, irksome (if necessary) regulations, and dead-looking coral where white hairy legs seem more plentiful than fish. Check out the dreamy view from the free gardenscaped grounds above the bay, and move on.
Alternative: Bactrack toward Kahala to Cromwell's Cove-Ka'alwai Beach, or continue past Hanauma to Makai Research Pier on the southern end of the Windward Cost. Cromwell's is a local's hangout on weekends, and Makai Pier, though hardly a secret, is no fuss.
Popular: Tunnels Beach, with the Bali Hai (Makena) Ridge lofted above, is a Hollywood fantasy of tropical snorkeling. But with limited parking, the fantasy turns to a b-movie. Same goes for awesome Ke'e Beach, at road's end. Too much of a scene at peak times.
Alternative: Backtrack to Princeville and take a short steep walk down to Hideaways or Kenomene Beach. Princeville is a resort area, but these beaches are fairly wild. Or go farther down the coast and check out hike-to Papa'a Bay. There's a movie director's mansion at the backshore of Papa'a, but the bay is postcard pristine.
Popular: Monkey-see-monkey-do tourists would hobble over piles of jagged lava to get to Kalaeloa Cove (the Aquarium) in the Ahihi-Kinau Natural Area Reserve south of Wailea. Due to adverse impacts, the government boys have shut this place down for a rest, and the crowds gather at nearby Dumps and Ahihi Cove, at the other end of the reserve.
Alternative: Not far away on this same coast are Oneuli (Black Sand) Beach and Chang's Landing, a.k.a. Turtle Town. Snorkeling tour companines bring boats from Kihei to these spots, which can be accessed from the shore. Oneuli has a sandy entry, while Chang's requires a nimble entry at a reef.
Popular: At the south end of Ali'i Drive in Kona, Kahalu'u Beach Park can seem more like Coney Island. Just too much flesh and hubbub.
Alternative: Head a tad farther south on Ali'i Drive to Keauhou Bay and the Keauhou Sundeck. Keauhou has tour boat launches and locals in kayaks and outrigger canoes, but relatively few swimmers. Or go just north of Kona to Old Airport Beach and make your way to Kaiwi Point, an excellent choice when surf is low.
These are just a few of many places that can be "best" for snorkelers on a given day. Bottom line: check out the Trailblazer guide for the islands which has all the spots described, and make the call for yourself.