On Kona it's the Manta Night Dive. Either Kona Diving Company or Jack's Diving Locker do it - so do most of the other operators. For a shore dive it's probably Place of Refuge. Second might be the sites near Puako. Info on shore dives for every island:
Scuba Shore Diving Region: Hawaiian Islands
On Maui it's Molokini Crater, Ed Robinsons, Mike Severns, B&B Scuba, others all do it. In West Maui it's the Cathedrals at Lanai. Lahaina Divers or Extended Horizons go there. Lahaina also does Molokini every other day if you stay in that area. Most of Maui shore diving is shallow. Best is hard to say but the better dives are probably Black Rock, Mala Pier, possibly Kapalua Bay. In South Maui probably Makena Landing or 5Caves/Graves. I personally like Wailea Beach but it won't make most people's top 5.
If you get to Kauai - the signature dive there is Sheraton Caverns. One of the best dives in Hawaii for sheer thrill is Vertical Awareness off Ni'ihau. It's an all day rougher boat ride though - only possible in the summer. I think they want an advanced card also. Or at least a decent # of dives. The best regular shore dive is Tunnels Beach on the north side - get a guide to see the best of. A decent shore dive on the south side is Koloa Landing. Either Fathom Five or Seasport Divers do all these.
Maui has the most diving potential. You've got Molokini 15-45mins. away (depends where you leave from) Lanai about a 30min ride - besides the Catherdrals there's some drift dives off West Lanai - a couple your g/f could stay in protected water nearby, all the decent shore dives plus you could go look for the Hammerheads off Molokai with Lahaina Divers. Might be too advanced for your g/f.
Whether Kona diving is better is subjective. It's the only place my buddy dove with several hundred spinner dolphins. Most of the shore entries on Kona are rougher - mostly lava to the waterline - or longer carries from the parking area.
One of the best dives of my life has to be the West Lanai drift. Our DM estimated that we saw 5000 (thousand) Moorish Idols holding in the current there. An equal # of tangs nearby - Yellows and some surgeonfish. And some black tips to keep it interesting. It was some years ago so IDK if that's changed. For pure fish life it's hard to beat Molokini - the snorkel boat feeding helps that. There's also a Manta cleaning station off Maui so it's possible to see them there. Nowhere near as dramatic and exciting as the Kona night dive though.