Is that a rhetorical question? Cause btc itself is a investment, you don't need to invest it. You don't buy gold and invest, then ask where to invest gold. Bitcoin don't need further investment, while you could buy and sell weed if you are in USA states. Maybe open a business with it.
While bitcoin is already an investment on its own because of its speculative and volatile nature, nothing says that it cannot be channeled into other means of production (invested) to create more of it. Just about anything that is an asset can always be invested and that includes gold. Gold can be used as equity contribution in a start up company, it can be lent out, it can be deposited in a bullion, etc.
As for the ideal place to invest bitcoin, honestly there is none in sight at the moment, because a lot has happened in this industry to suggest that we seem to have much more rouges/scammers in the cryptocurrency industry than legitimate entrepreneurs. As such the best bet investment eventually boils down to buying and holding your bitcoin and in a cold wallet, waiting for the price to appreciate, which it does, from time to time.