In a fight who would win?

Sans or Omega Flowey?

Flowey. Despite Sans’ battle prowess, his drive to protect his family, and his utter loathing of the weed, Flowey is still in charge of resets. Only an equal or greater power could stand against those and hope to win.

Sans would fight to the end, until he was broken down and could do nothing more, but an end would come eventually, and it would be a bitter one.