No, the legal versions of Obelisk and Ra do not have serial numbers on the bottom left. There is no legal Slifer yet.
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That's the number you get when you pay for the software that allows you a legal right to use it.
You can not physically make your own YuGiOh cards that are legal to use in any sort of game. However, you can make your own gag cards for comical reasons. Instructables has a tutorial on how to make them using an image search.
Dark Magician of Chaos and Magician of Faith both had this power, but they are currently both Banned. The only legal card I can think of off the top of my head would be Magical Stone Excavation. Also Z-One
In the show? No. Technically, the creator of light was never used as a card. In the game? Once again, no. As far as I know, the creator of light was never made a card. The strongest card overall in my opinion is Winged Dragon of Ra, however, for "Legal" cards, Either Dragon Master Knight, or Five headed Dragon are strongest single cards in terms of attack points.
Depending on what you torrent it is not legal there are a few legal torrents out there but most are copyrighted and therefore illegal