Email services in general. My worst nightmare is my email provider (which isn't Google) going dark and losing access to everything.
If you use a password manager like Keepass, you should still be able to log into your other accounts if you lost access and at least with financial institutions you can call, ask that no changes be made with without coming into the branch and showing ID.
You can use a custom domain with most providers, so when they go dark you can at least migrate to another one.