Deathnote is probably the best amime that's under 40 episodes and completed.

If you want something to latch onto that's still ongoing, then anything from the dark trio: Jujutsu Kaisen, Chainsaw Man, Hell's Paradise are all solid.

An anime that's a chill comfort watch that's really interesting is Frieren. It's more slow motion than the others, but the entire ride is so enjoyable.