I always try to avoid "there is only one". The problem with that being that it shouldn't be a mystic thing; i.e. you trigger correlation with "the one true faith", "the one to rule them all"... and that historically has never stood the test of time. It's not a domination thing because Bitcoin isn't a government; it's a protocol.

Instead, if someone insists on their shitcoins, I explain that Bitcoin is the only thing out there that wasn't made as someone's plan to get rich (it's easy to prove this for anything that premined/ICO'd/dev tax), and the protocol today really only built on top of the original, apart from some early bugfixes, which basically takes care of the few remaining shitcoins that weren't meant to make someone rich.

I also try to steer into a direction where there is talk about how no one represents Bitcoin. So no matter what some influencer / president / rich person or even a retard like me says, Bitcoin is no more than the protocol and it's the only thing we all agree on. It's just alternative p2p money where no one is in charge. This brings much more fundamental discussions, and will scare everyone away that's looking for a quick buck.