Mine is not ULTIMATE, but it was an epic adventure for me.
I enjoy trips when the group is 2 or 3 max, and buzz kill for me are crowds on a trail, or summit.
In Southern California, I've contoured figure-8 Santa Catalina Island, in 2000 and 2005. The ocean, sky, clouds, sunset views are great photography, the inside of the island though was brutal. We even compared it to being more strenuous than Whitney or Gorgonio. It was the expectation. Thought it would be a cake beach walk, didn't realize that every half mile was 600 ft elevation gain, loss, then gain again. I got solitude in the island backcountry, the occasional roaming bison. And it's an island, can't get lost, though many map trails were washed out, that was a plus for stealth camping away from the rangers and tourists in Avalon and Two Harbors.
If you want even more rustic islands, look into Santa Cruz Channel Island. I've done it in 2009 and 2012. Start in Prisoners Cove in the middle of the island, and bp to the other tip Scorpion Cove. You cross 5 climate zones. Beach, coastal bluff, pine, desert, oak, farmland. There is historic points of interest, as well as some animal natural selection that would make Darwin jump for joy. Option for cave kayaking.