First off , Bitcoin will always have value, even when it will not be no1 in marketcap , or be used daily in everybodys lives, in 200 years it will at least be part of someone's coins collection. As I am sure many will be lost due to mechanical failures, or just plain human stupidity, it will become rarer and rarer.
As far as marketcap, I am sure Ethereum will take the crown, maybe as soon as early 2019. Only because of the sheer number of ico's coming out now based on ethereum. And if the integration of POS turns about to be succesful ..boom.
Will ether keep the crown ? Man who knows ? Before 2009 there was no Bitcoin , maybe in 2020 we will have no money anymore because we will all be controlled by the AI -blockchain.?
In the end, it comes down to what everybody thinks will be the best. Right now a lot of people think Bitcoin is it. As crypto will see mass adoption , bitcoin will be evrybody's first, because they have all heard about bitcoin. If the majority of the people decide to buy tomorrow, and someone tells them it will be better for them to buy ethereum, they'll think that that person is trying to con them " Ether ..what ? " Just try talking about crypto to..EVERYONE you meet , EVERYWHERE .