The best JavaScript libraries supporting Bitcoin Core?
I highly recommend bcoin:
https://bcoin.io/
https://github.com/bcoin-org/bcoin
It is a nodejs library that can also run a fully validating node, SPV node, and BIP44 wallet. It's used in production by many long-standing bitcoin companies such as Purse.io
You can even use the bcoin client to make RPC calls to bitcoin core, and of course you do not need to run the bcoin full node to take advantage of the library for TX construction and wallet functions.