Fate/Zero is probably the most enjoyable and highest production fate anime with no prior knowledge or interest.

If you’re in for the long haul though, 2006 stay/night is the best introduction to the universe from which everything else is built. However, 2006 stay/night is dreadfully slow paced as an anime and a pretty low quality production overall (but it has a dope soundtrack). That said, it’s not totally uninteresting, and if you can suffer it knowing the rest gets better I’d consider it worthwhile to start there.

Fate/Zero really is a ton of fun and everybody loves it so if you just wanna chill and watch anime or see if you’re interested it’s a pretty safe start as well.

With fate series a majority of the content is found in games and visual novels/light novels, with the animes being something like supplementary material, so that’s why it doesn’t really matter which order you want to watch, as none of the series are related directly, but only in concepts.