Monero, the Bitcoin supplement

Monero is the top privacy coin. Monero is what Bitcoin noobs thought they bought. While in the past not being the best store of value, its strength lies in its payment function.  You don't need privacy until you do, and then it's too late. 

Yes. They are legal in most countries, but there is a risk that they'll be banned in the future. The circumstance that Monero is also used in illegal activities  does not make legitimate transactions illegal or even unethical.

Its characteristics are stealth addresses, ring signatures, zero-knowledge proofs (zk-proofs), and RingCT form Monero's proprietary privacy protocol. There is no evidence yet that any transaction has ever been tracked and traced. However, certain governments and contractors are trying to develop tools to help tracing Monero. The future is uncertain and new tools might be used for old transactions. 

Monero is increasingly being used as an intermediary for Bitcoin transactions. Bitcoin holders buy Monero and use that currency to settle transactions in order to complete private money transfers without having to expose their public wallet address. The receiver sells the Monero against Bitcoin as store-of-value coin. Buying and selling can be accomplished on an atomic swap without KYC requirements.