None of the six best answers hang with traditional investments unless you manage some scorching low buy-in price which would typically be the result of someone's error, something "not legal" or some random long shot.
For someone's error, find a widow or an heir if you feel comfortable doimg such things. For "not legal", we don't discuss those on this site. For random long shot... lottery tickets are maybe a better buy.
Best three would be bespoke guns of limited manufacture, such as Fabbri, Korth, Hartmann and Weiss, etc., whether shotgun, rifle or handgun. Only issue is their starting prices.
Want to make a million dollars collecting guns? Start with 2or 3....