This is a simple answer. Rurouni Kenshin.
No matter how many times I’ve watched Rurouni Kenshin, I never get tired of it. Especially the Kyoto Arc. That is probably my favorite arc in all shonen, because it’s just too perfect. In general, Rurouni Kenshin has a great and loving cast of characters who are very well layered, a great story, a great premise, and great action scenes. I don’t feel like I need to explain anymore about it. If you have never watched Rurouni Kenshin before, please do. It’s just so good.
I almost put down Yu Yu Hakusho, because I can say so many things about that series as well, but Rurouni Kenshin edged it out.