!!! SPOILER ALERT !!!
Perhaps, but I could also argue that Sephiroth constantly has his facts wrong in the FF7 storyline, as well as a complete idiot for "toying with Cloud" despite having a record of 0-4 or so against him. But that's what creates the character. In art, flaws create perfection. That's why I liked Yu Yevon, who was so desperate to cling to Zanarkand that he created Sin and forced the fayth to summon a dream version of his former home, then demolished any sign of advancement to protect Zanarkand. In that sense, it blurs the entire concept of good and evil. He wasn't a God, he wasn't trying to be, yet he had a much longer lasting and more devastating impact in his world than almost every other FF villain.
I said not necessarily my favorite because it's a toss up between him and Kefka, who I honestly just found hilarious in his own odd way. No further explanation needed, really. Same with Sephiroth, who I can understand why he's liked and all.
EDIT - Apologies, obesebear. I'm not sure if you were sarcastic or not, but regardless I should have added the spoiler alert. As cool as I thought Sin was, and the back story for FFX, the game itself had a lot of things going against it. Namingly Tidus.