This depends on the person. If you think any character, even Dragon tiers count, then the best one with a video would be Giυgas by Helltoast. It's full capabilities haven't been caught on video but apparently it can even run on a computer that has been rendered inoperable. There's characters like Katsuragi, who apparently disrupt your router and Wi-Fi, but there is very little info about this character. If you mean characters who don't use external files or disrupt the computer, then def-test by ydccdy and Zαrατhuστrα by Ced Soum (characters who only work on XP) would be the strongest since they K.O you the moment you open M.U.G.E.N. There's also characters like G A S T E R by Ignacio Martinez aka Kingdemonic666 or Michael 2.0 by Ced Soum who basically have their own M.U.G.E.N, along with characters who run when the computer starts up. If using glitches or exploits don't count for then I honestly have no idea, there's just too many characters that you lose track.