Ethereum

The name "Cryptocurrency" is a misnomer.

Cryptocurrencies are not actually currencies, even though they are based on cryptography rather than violence. 

They're crypto-based versions of precious metals, bonds and stocks. They should actually be called Cryptometals and Cryptobonds and Cryptostocks.

We wanted eternal life, instead we got 140 new genders

Why is it hard to make a Cryptoasset that can be used as currency? 

Well, for one thing, there isn't actually much demand for it! People don't need a Cryptocurrency. Fiat currency works, mostly. 

What people really need is Cryptometal.

Cryptometal is a way to hold something like gold, without the fuss of actual physical gold bars that are annoying to assay and heavy and bulky and slow to transfer to other places. "Gold but without the downsides of physics" is incredible, it has gone up by something like ten million percent since Satoshi invented it

OK, but what if people did want a Cryptocurrency rather than a Cryptometal or a Cryptostock? Could we make that? Well, yes we could. Right now people mostly use fiat-backed Cryptoassets called stablecoins as a medium of exchange that doesn't fluctuate with the overall risk appetite of the market. Tether, USDC and so on

There are also some unbacked or algorithmic stablecoins, but those can implode violently (see: Terra/Luna) or they can suffer from various scaling problems as it is hard to scale a non-backed, stablecoin to a large market cap.

Personally I rather like @ameensol 's RAI/HAI projects  for a cryptocurrency. But note that these are currencies, not metals or stocks and so you should expect to lose value holding them, just like you lose value holding any fiat currency.

The point is you lose value very slowly, and sometimes that's great (when every other asset is losing money very quickly!) 

$RAI is the only actual Cryptocurrency!

An actual crypto-currency is the hardest crypto-asset to make and so far nobody has actually made one. Well, there were attempts like $DAI before it added dollars as collateral, and $RAI that has very little adoption (and a future clone of RAI called HAI might be the most interesting attempt at cryptocurrency).