Proton Mail with a custom domain. The only reason why is that I had it before I knew Fastmail existed and it would be a pain in the ass to move my entire family to it. However, I was VERY tempted when 1Password put Fastmail temporary email support into their product.
Fortunately, Proton Mail just released their own temporary email product based on SimpleLogin.
The only true failure is not trying. Everything else is a learning opportunity.