Monero. You can even send a transaction and it will appear as a "cero-monero transaction", even when coins have been sent.
Not for nothing, this is the favorite coin to the black market.

As you can see in the link below, monero transactions can be confidential, only the person who send the money and the one who receives it can know how much money the transaction has: https://moneroblocks.info/tx/8da468ec3d17f38a452783441772341c51cf56e6ef2d2d7fd74ccea347253538

So, if your question is about confidentiality and privacy, monero is the principal altcoin where you can hide how much coins you send.