It kind of depends on what you like to do. Each island is a little different.
My husband and I really enjoyed the Big Island and had a blast exploring the national park. But the Big Island is not really a beach island as it is quite rocky and sandy beaches are a bit difficult to get to. There are some fantastic snorkling spots.
Maui is lovely and is much better for beach sitting and swimming. Snorkling is good here, too.
Oahu is very popular and more crowded than other islands, but is good for beach, surfing, snorkling, etc.