Send me a crystal and I'll tell...
Aquamarine: Where my muse feels most calm/relaxed
I would have to say either the Secret Place or the mini island with the Paopu Tree on it.
In the Secret Place he is often never found, and he can just kind of sit in there and quietly listen to the wind blowing through the open ceiling and reminiscing. In the small cave he can just pretend that nothing ever went wrong-- that the islands never fell, and he was never given the keyblade. That everything was just... Peaceful. The Secret Place is where one should look if Sora simply just... Disappears.
And the mini island just allows him to watch the sunset from the Paopu Tree trunk, just as he used too with Kairi and Riku. He loves to listen to the waves hitting the shore. He finds it incredibly relaxing, and it’s here that he goes whenever his nerves are acting up.