What cryptocurrency has the most potential to replace everyday cash / casual transactions? I am asking in terms of transaction fees, scalability, transaction time etc. I am a bit lost with so many cryptocurrencies coming out and focusing on different problems. What's some of them or one of them that meets the requirements of replacing everyday transactions? Or maybe even one that could be transformed to deliver that.

Thanks for your answers in advance.
No blockchain that handles the world's daily transactions can scale because of the sheer size of the blockchain required.
Something without a blockchain (e.g. Mimble-Wimble) might be good enough.

So, for blockchains, the current thinking is that another layer needs to be used. Bitcoin is the most advanced in providing ways to do this: segwit (deployed), lightning network (testing on main net started), drivechains, sidechains etc. This combination looks to be the best design for scale while optimizing cost, security and decentralization.

In the short term you might try faster blockchains like Dash or Ethereum. In the longer term there's no reason to believe these will scale securely though.
You can safely disregard 99% of the alt-coins out there. They're just there to grab some money from buyers.