Basically any upbeat song that reminds you to appreciate life in the middle of absorbing and applying theory. Anything from the Internationale to Red Army tunes to modern(ish) artists like Chumbawamba are allowed. Even Lofi.
Personal favorite of mine is Quand Fera-T-Il Jour, Camarade? by Mireille Mathieu. It's bittersweet, hopeful and resolute. Even if I don't speak French, I sometimes mumble to it.
Runner-up would be Solidarity Forever by Pete Seeger.
New Phone Who's This by Georgia Maq (of Camp Cope) is a good modern take on this kind of song. At least for me it addresses a lot of anxieties present in our society and provides a hopeful outlook.
People by AJJ is a good representation of a feeling I think all leftists have. Of recognizing the shit that people do and how terrible they are but ultimately believing in the good in people and the love inherent to being a leftist.
People probably put me on lefthand path tbh