It will always be .com.
.co.uk / .de / .fr / .cn and other popular geo extensions will be equally as trusted.
.org for non-profits will be equally as trusted.
.net has been around forever and should probably be up there but I'd place it slightly lower. Just like with .co, people must wonder "if this is such a trusted company/website - why couldn't they get the top-level domain extension?"
.biz has been around just as long but still seems sketchy - I suppose because you don't call a business a bizness.
.co is technically Columbia's ccTLD but they're making too much money to care and tell customers to register .com.co for Columbian businesses. Also, I've always thought it was perfect for those with ill-intent hoping for visits from popular.com's to their .co's or even private emails that missed out the "m" at the end of .com by mistake. Although this article disagrees that .co domains are worthless (and they have some really great graphs):
https://growthbadger.com/top-level-domains/
This comment sums up my thoughts:
"It’s hilarious to me that people trust .co – a country code top-level domain for Colombia – more than they do .org – a generic top-level domain used by Wikipedia, non-profits, and other orgs."