What currency do you think has the soundest design? I know Bjarne Stroustrup said that he doesn't like bitcoin with one of the reasons being it is so computational heavy(bad for the environment).
IMO they all suffer from scalability problems, in that the blockchain grows basically without end. Blockchains with smart contracts have the added cost of needing SSD to finish syncing in reasonable times (Ethereum needs somewhere close to 1 TiB of SSDs, IIRC)
I think the ones offering strong anonymity like Monero etc. are best. I'm still trying to find a good explanation of how ring signatures are supposed to work.