I hard to say a single coin is that is going to perform the best over the long run, because honestly the best strategy is to diversify your investments into different cryptos, and hold all of these coins, to reduce risk and maximize your profits.

If you want the best long term investment to buy right now, then I'd say IOTA. But that is taking into account the fact that its price is quite cheap right now.

The most promising long term project is still ETH, in my opinion, although its prices have already risen so much that it may not be a good investment.