You may or may not get serious answers, because you're asking people to tell you where to find the golden goose. The trick of timesharing (as I'm sure you know) is to make the most of what you own, with the least amount of investment. If the secret gets out of the hidden gems, they won't be hidden any longer, and the value could easily slip away. Great trading timeshares are usually those located in higher demand areas, but with lower density of available timeshares. The trick will be to find one that everyone wants to visit, but where there is less inventory to satisfy the demand. And unfortunately, those places usually come at higher cost, which offsets the resale value for the new buyer. A seller who owns something they know is great isn't likely to give it away too cheaply.
I'm not a Marriott person, so unfortunately I can't speak to your exact request. In my limited experience, every Marriott is a good trader, for the reasons I named above.