If you've never been....Oahu is the action Island with lots to do for a first time visitor. Everyone should go there at least once. If you want seclusion or a quiet honeymoon spot...Maui or Kauai would probably be an ideal spot. Unless you're staying longer than 8 days...Island hopping can be tiring and provide only limited time on each Island. If you're going for 2 weeks or longer then Island hopping probably wouldn't be a big issue.
I've been in December, May and September. My favorite times were December (had Oahu almost all to ourselves), and May. June - September is really peak season, both crowds, and expense wise. You'll get better financial deals in off-peak times, and the weather is milder (80's).
I've stayed at the Outrigger Reef and The Hilton Hawaiian Village on Waikiki (Oahu). The Village was my favorite. It's a paradise in itself.