I haven't had a new favourite song for a while but "Biidama" by Saori Atsumi is quietly slipping into that position. It's performed in Japanese so there's no way I would have ever heard it if it were not over the closing credits of season one of the Genshiken anime. So it's not new -- 2004. I think the thing that makes it work is that, with its nostalgia for perhaps unhappy times, and its waltz time, it's actually a French chanson. You can listen to it here three times if you need an earworm. These are the lyrics in Japanese and English. Here is a list of other songs of hers that have been used in anime. I would very much like to hear it sung in French. D