Popstar - Hoshi no Kaabii
It's slightly sad but so beautiful, and it hits me hard in the feels every. single. time. It always gets me in a sentimental mood... but it also reminds me of all the sad episodes of the anime it was used in, which makes me tear up because of that as well. To me this song is beautiful, sentimental, happy and sad all that the same time, but I love it so much.