I liked all three islands I've been to (have not been to the Big Island or Lanai or Molokai).
My DH's least favourite was Maui. I liked it but did feel a bit like you were sort of stuck in a resort (and typically one of maaany resorts in a row) or a condo with not a lot of other great choices. To me upcountry Maui was the best part, just lovely. It's a good place for honeymooners but for me I preferred the beaches on the other islands.
I actually liked Oahu more than most people tend to but that's because I only bothered staying in Honolulu for a couple of nights, just so we could go to Pearl Harbour and so I could experience Waikiki and go hike up Diamond Head. Needless to say, would not stay there again. Waikiki is basically representative of what I hate the most about European beaches (sorry Euros, but I did live there for 10 yrs) - it's essentially towel next to towel so you feel like you are right on top of strangers. And we went in the off season so I can't even imagine what it's like when the hotels are fully rammed. What I did like about Honolulu was Pearl Harbour (really excellent memorial, good museum, everything about it is well done IMO), the cemetery of the Pacific (beautiful, peaceful and gorgeous view) and the "hike" up Diamond Head, I say "hike" because it's kind of like a bit of a harder walk up the hill lol. Beautiful views and worth going. I also did like that Honolulu was urban so after a couple of weeks of very slow living on the islands as a city girl I just craved a bit of humanity and good shopping (of which there is lots!). On Oahu I've stayed in Kailua (just down the street from where the Obamas typically stay) and in Lanikai. I cannot stress how much of my heart remains there. I just LOVE this area. The beaches are in my top 3 in all of Hawaii, especially Lanikai. It's magical. You're only about 35-40 mins from Honolulu so not like there is a long trek to the airport or the "action". Kailua is a beach town but large enough for major grocery stores so I think you'd have no issues. Also good restos. Just basically perfect in terms of size and things you need. I did like the north shore but personally would not stay there as it just didn't click for me. Stay away from those weird man made beaches on the other side of the island (where the Disney property is), that's just odd if you ask me.
Kauai is sleepy, wild, gorgeous beyond belief. To me it's the most beautiful island by FAR. There is nothing and I mean nothing that compares to the Napali coast. When it is a perfect sunny day on the North Shore I think nothing comes close. Would highly recommend staying in or around Hanalei. Hanalei Bay is also in my top 3 beaches, the backdrops of the mountains are stunning, the crescent is perfect, and you can always have lots of sand to yourself. There is a grocery store there, not a huge one but should probably be ok. There is also a fabulous Costco in Lihue by the airport where you can stock up (and other big stores). If I win the lottery I am buying a house in Hanalei, done deal. The only "downside" to Kauai is that the North Shore is the wettest place on earth (which is also why it may be the most beautiful). So you will get gorgeous weather in the summer but it could still rain a bit. In winter it is pretty wet up there. The south coast around Poipu IMO is not great and I would not stay there - too touristy, too resort-y and too unKauai to me. It's closer to the canyon and a number of the Napali hiking trails but those are just about the only redeeming features IMO aside from the amazing, hot, dry weather.