Never been to Kona but heard is very nice. Been to the others and found Kauai to be the most relaxing. Since you are looking to stay at a spg property, I recommend either the Princeville Westin or St Regis over the summer (best time of the yr to visit the North end). They are within 3 miles apart and the views can't be beat. Especailly from the St Regis. I stayed at the Westin and there is a hourly shuttle to the St Regis beach.

From my own personal experience, I like the Westin becasue is a timeshare where you have a kitchen/WD in the room. We also really enjoyed the outdoor gas grills which we took full advantage of. It also won't break the bank as compared to the SR.

I also stayed at the Kauai Grand Hyatt and I actually perferred the Westin because of its location. GH location was not as scenic and the hotel was a little too busy for me but the kids loved the pool.

From Westin Princeville lawn


From St Regis


Beach at St Regis


Queen's Bath near Westin in PV


From boat off the coast of Kauai (Napali coast)


North Shore Hanalei Bay