Poipu Beach on Kauai or the Wailea end of Maui would have to be at the top of my list.

Both of those locations give you an excellent chance of perfect weather and a great sunset. Same goes for Four Seasons on Hawaii - I just personally find Maui and Kauai to be more romantic than the Big Island.

The Kapalua end of Maui has a higher chance of clouds/rain. Don't know about Princeville but I know most other parts of Kauai have a higher probability of rain.