We just did six days on Kona, on the dry (west) side of the Big Island. If you're into diving, Kona is great -- the manta ray night dive is not to be missed. Kona is also much less commercialized and developed than Honolulu, which has 8-lane freeways and feels more like L.A. than anywhere else.
The Big Island also has the active volcano, plus you can take a trip up to about 13,000 feet (Mauna Kea). Some great beaches all along the west side of the Big Island, especially north of the Kona airport up towards the northern tip of the island.
Haven't been to Maui or Kauai, but Kona was a great trip for us.