I use the one that’s built in to the Fastmail service. I have a custom domain just for aliases. The Fastmail alias-creation API is integrated with the Bitwarden app (which I use) so that makes creating new accounts (that use email addresses as usernames) on websites really easy. I also use Spamgourmet which is free, convenient, and has been around a very long time. No custom domains there, but they let you use a variety of their domains and they have some short ones which is nice, but I do find that they’re blocked pretty often, mostly by major mailing list services.