To add to this, Bitwarden integrates with a few of them so you can automatically generate a unique email on the fly when creating an entry for a site. (docs link)
I use Fastmail and use my own domain for about 90% of stuff. Sites I don’t even want to have my domain get a random.gibberish@fastmail.com email autogenerated from the BW extension. It’s pretty convenient once all setup.
For literal one-time use, I’ve generally had good luck with 10minutemail. I’m not sure how much it gets flagged as spam right away these days, but the nice thing about it being free is you can always try and find out for a particular site.
What are the odds there’s at least one reddit employee’s IP on the list?