On Big Island is Hawaii Botanical Gardens at Onomea Bay, just north of Hilo. Jungle setting, waterfalls, oceanfront. Note; The trail runs 500' down from the parking lot to the ocean, is steep. There is also the Nani Mau Gardens south of Hilo, has a tram ride, but feels more like somebody's back yard than a tropic garden.

If you are in Oahu on May 9 you could visit Oahu Urban Garden Center.
Here is what it say on their website:

Lei Day in the Garden!
Make Ti leaf leis and Ti leaf rosettes to take home. Exhibits on
lei plants and how to grow them in your home garden! Special
presentation by The Garden Hula Dancers!

Which are the best depends on what your particular interests are. What do you want to see?

Thanks for the suggestions leilaniguy and randg. I will be visiting May 17-23, so I will unfortunately miss the special event at the Oahu Urban Garden Center.

bboy - I am most interested in Aroids, but I know there are not a lot of Aroids that are native to Hawaii. I will look forward to any type of tropical plants that are viewable. If you have a favorite, I'd like to hear what it is.
Thanks!

This collection contains spectacular aroids.

Here is a link that might be useful: Foster Botanical Garden, Official Web Site for The City and County of Honolulu

this one features an area designated specifically as an Aroid Garden.

Here is a link that might be useful: Wahiawa Botanical Garden, Official Web Site for The City and County of Honolulu

I think you've possibly made a huge mistake in not visiting our Garden Isle of Kauai. The National Tropical Botanical Gardens here in Lawai is very simply one of the best in the world. Also you see "large and live" versions just driving around our Island.

Aloha and mahalo

I've only lived on Oahu for a year, but I've enjoyed the Waimea Valley botanical gardens. A beautiful walk...

Here is a link that might be useful: Waimea Valley

Lyon Arboretum on Oahu, which is only 2-3 miles from Waikiki if you are staying there.

Here is a link that might be useful: Lyon Arboretum

In Hilo

Queen Liliuokalani Garden

In Kona

Hawaii Volcanos National Park

In Puna

Lava Tree State Park

And Onomea Bay as well... :) Hawaii Tropical Botanical Garden