A .com is always the first preference. But the perfect .com domain name that you come up with on the first attempt would already be taken by someone else. At this point you can explore other TLDs like .co, .org, .io, .app, etc. Despite being the most popular TLD, a .com gives you no special advantage over other TLDs in terms of SEO, reach, or any other matter really.
The renowned design studio UENO chooses to use ueno.co instead of ueno.com (they own both the domain names). AngelList chooses to use angel.co instead of angellist.com. Craigslist, Telegram, and Wikipedia rock their .org domains with zero issues.
If you’re setting up a new domain name and a .com is not available, do consider looking at other TLDs before dropping your choice. Alternatively, try entering your keyword into our generator to get many alternative name ideas.