It's between speed vs. decentralization. Which coin will be adopted by most? Many say it's in the top 20 already we have still decided what we're going to use as THE PROTOCOL coin. Educated opinions welcomed.
I think it's more complex than that. There isn't one all-purpose coin that can be used for everything. For examples in the top 5, ETH and XRP are completely different projects with completely different use cases.
I do think a fast payments coin will be big, very probably XRP the winner. At least one smart contract coin will be big, probably ETH. But there are way more uses and coins to suit them - IoT for example will likely have really taken off by then, so Iota or a competitor. DAG projects such as Raiblocks/Nano could well be big, Hashgraph might be viable, and on and on.