Honestly, for me, any time is good to visit the islands, but during the summer it can be pretty hot. probably around 90 degrees.
Which island you want to visit depends on what you're interested in. I only have experience with Oahu and Maui. Both have a different style.
Oahu has a larger population living there, still plenty of outdoor activities and good food, but its more like living in say Los Angeles.
Maui is more laid back, plenty of stuff to do outdoors and in my opinion the beaches are better. Definitely less populated and lots of places close at 9PM.
Im personally thinking of visiting the big island my next trip. I may fly into Oahu and take an inter-island flight to the big island for a few days.