となりの関くん 1

The anime of this series has long been a favorite of mine, so it seemed like a good idea to tackle the manga since the setup is pretty simple. Each chapter Seki-kun devises some new and intense method of keeping himself amused, while his studious neighbor tries to get him focused but ultimately becomes swept up in his strange imagination games. I do think it works a bit better as an anime since you have tighter comedic timing and the anime had some amazing sound design, but it's still a ton of fun to read his hjijinks and the manga has lots of antics that never made it into the show.

Language-wise this is a fairly easy read, but the diversity of Seki's interests means you'll run into a good variety of niche vocabulary relating to hobbies, arts, and tools. The two bonus chapters at the end are a little tougher as they rely on some strange wordplay and have handwritten kanji, but on the whole this one is pretty accessible.