I agree with both @Unboxious@ani.social and @falidorn@lemmy.world regarding the definition of a must-watch anime. There’s nothing that can please everyone, and yet there are some things everyone should have at least heard of. Hearing of something may be a low bar, but it is the precursor to attempt watching it.

To put it another way, this question is asking for the greatest (read: most popular) of all time for anime.

Wikipedia has lists of GOATS for various media:

• highest-grossing films
• highest-grossing Japanese films
• best-selling books
• best-selling manga
• highest-grossing media franchises

Some examples to save you some clicks:

• Mario
• Pokemon
• Star Wars
• Harry Potter
• Avengers / MCU
• The Lord of the Rings

Interestingly, even though we’ve got a list of best-selling manga, we don’t a list of top anime.

However, there are heuristics - we can reference the list of highest-grossing Japanese films and most popular anime on AniList (or myanimelist, etc)

Some picks are:

• any of the big 3 long running Shounen series: Bleach, Naruto, One Piece
• Pokemon
• Attack of Titan
• Demon Slayer
• Your Name
• Ghibli films

I suspect Frieren may join the ranks at some point.

Personally, I have not watch Bleach / Naruto / One Piece and dropped Demon Slayer.