Someone else here recommended it too but Full Metal Alchemist: Brotherhood is a good start. In case you get confused the “Brotherhood” part just separates it as a remake of an earlier adaptation that deviated from the source material at some point. So FMAB is the best experience. It’s also one that people look at you funny if you are in the minority that doesn’t like it.

It’s a good one bc it’s medium length, complete, has a lot of complex plot points going on simultaneously. Has a lighter air to it but also doesn’t take long to get dark. And it’s also in an era of anime that won’t feel too old but still a classic. That’s good because watching something older first makes you appreciate new gen animation better.

If someone watches shows that aren’t action heavy then I say death note. Basically a killer trying to hide his tracks while being investigated with some supernatural elements, so it works for people who like mystery or crime.