There are a couple I have in mind. Like many techies, I am a huge fan of RSS for content distribution and XMPP for federated communication.
The really niche one I like is S-expressions as a data format and configuration in place of json, yaml, toml, etc.
I am a big fan of Plaintext formats, although I wish markdown had a few more features like tables.
RFC 3339 is a simplified profile of 8601 that only covers YYYY-MM-DD style formatting, if you only ever use that format and avoid the things like “2024-W36” they’re mostly interchangeable.
The week-of-year is far more relevant in Western Europe, and is used quite a bit in business. I have a Junghans watch that has a week complication.
It’s an important format outside of the US, and gives ISO-8601 an edge as a standard of conformance.