.ai is also kind of cool right now but it will probably lose its appeal after some years when everything will become AI and the world will get tired of the term.
But .com is still at the top of the food chain. If you can survive for a few years, raise your series A/B, you will likely have enough money to acquire your sexy single-word .com name.
I’m still annoyed that thanks to a missed payment, I lost quixote.com.
For years, Twitter used ".ly" and it was not widely known that this belonged, ultimately, to Libya. The other ccTLDs, such as .io and .co, have certain "nerd cachet" and people think the occupants are cool, until the rug is pulled and they need to reckon with the fact that all registrants were actually cybersquatters and victims of rampant capitalism.
My advice to any startup that can't get a .com, is to find a legitimate gTLD that you can occupy. Even ".us" is available to anyone in the USA. I do not understand the shame, or reluctance, to occupy space in ".us". Why is it so rare? Do you hate America? Seriously, do you hate her so much, you can't live here with her in cyberspace?
If you choose a ccTLD and you become attached to it, or your customers become attached to it, or your software is attached to it, someday you will need to face the music when Genovia is invaded, or transfers its sovereignty to someone who wants to enforce Genovian supremacy on the Internet.