Monero

They'll tell you that it's because the supply can't be audited, protocol upgrades occur on schedule and so the protocol is not decentralized, tail emission means uncapped supply and all this kind of stuff. And those may be points that merit discussion. But the real reason is that it's the one coin they can't mention when they say "shitcoin" because people will pick it over Bitcoin.

You can say whatever you want about altcoins, call them shitcoins. Most are. Say what you want about ethereum and the explosion of "crypto", a lot of it is warranted. But Bitcoin has entrenched interests that will never allow a protocol upgrade to make it 100% private, and so they know they cannot possibly compete with Monero. A transparent ledger is user hostile. Maybe it's their bags, maybe it's their tribe, but for one reason or another the maxis will shut you down when you mention Monero because there's no reason that stands to scrutiny why Bitcoin is better.