My personal favorite is INS/MV (stands for In Nomine Satanis/Magna Veritas). Base idea is that God and Satan started one big chess game called "The Great Game" when baby Jesus was born, using demons and angels incarnated on Earth as pawns, with the fate of humanity at stake.
Flash forward a few centuries, Lucifer is depressed, God is senile, and nobody really knows who's in charge anymore. High ranking angels and demons are as busy fighting their own colleagues as the other side, while low ranking angels and demons (the players) try their best to survive their superiors' agenda.

I would recommend the 4th Edition, which is out of print but can be downloaded legally here https://www.xxiemeciel.com/ins_ebookv4_01.php (first PDF on the page)