While I love One-Winged Angel and have nothing against it being used in other installments I think it gets way too much focus when compared to Those Chosen by the Planet, the actual character theme of Sephiroth, something that many don't even know which is even more proof how criminically underused it is
Those Chosen by the Planet really should get more love. It's so simple yet effective, and used in brilliant ways as well. It kinda has two layers, so when you're talking with Sephiroth before he goes full evil it's very understated, just a heartbeat on the drum and some chimes, then he storms out to find JENOVA and tear up Nibelheim and it kicks into the full theme, and it's damn scary.