Can't pick anyone below 126lb's. Assume McGregor natural fighting weight is 147 I don't think there is anyone... I wreckon Chris Algieri would knock the living hell out of him lol! Floyd will be sparring men who would KO him in 1 round. I wish i liked WWE.
I would pick any of the Micro Four to slaughter and KO McGregor --- Gonzalez, Inoue, Cuadras, Estrada.
That's going a bit far. I think McGregor would be a good domestic-European level fighter if he fully dedicated himself.