I think IOTA is pretty promising, as it is a cryptocurrency that runs on a Directed Acyclic Graph (DAG), and not on a blockchain.
The idea is that DAG can present solutions to some blockchain's issues. In a DAG network there should be no miners, no fees... And IOTA uses some kind of Proof of Work.
Is Directed Acyclic Graph (DAG) Blockchain’s New Competitor?But there is one thing that I really don't fell good about IOTA. It's about their Coordinator, which is like a Proof of Authority.
The Coordinator is a special node run by the Iota Foundation. It's main purpose is to protect the network from attacks, until the network is big enough to sustain a large scale GPU attack.
The Coordinator makes special transactions called Milestones, which are signed by the Coordinator and are "trusted and legit".
The problem is that now the IOTA network is no more decentralized than Ripple Unique Nodes. What if the Coordinator is compromised?
In future the Coordinator is to become optional or even useless, but as for now IOTA system looks like a centralized service. It's important that investors and users knows about this, even if this is information is polemic and controversy.
https://medium.com/@ercwl/iota-is-centralized-6289246e7b4dhttps://www.reddit.com/r/Iota/comments/7c3qu8/coordinator_explained/