Nano is a very interesting one. The foundation developing it focuses on one thing: developing the best digital currency. They don't implement smart-contracts to make it as efficient as possible for example. The feeless aspect is what makes it great but also maybe vulnerable at the same time: the foundation will have to look for external finance a bit like the Linux foundation of which they are taking inspiration.
I believe it has one of the best TPS for a decentralised system, and this can be improved with better hardware when the need for higher TPS comes. Now, as it stands today, you can transact on Nano with a 0.4s confirmation time, so higher TPS is not a necessity.
