When playing the game for the first time I hadn't been exposed to villains in the same way as RPG's feature them. So while I'm not on the Sephiroth train by any means, when playing the game he was mysterious and felt like someone/thing to be feared which left a big impression on me. I wanted to stop him, but at the same time felt unsure of what exactly was waiting for me when I would finally catch up to him. If i'm honest, i have never really thought about why he is popular, but I know I like him as a villain.