Ultra Sans is the final form of Sans. He can turn into Ultra Sans with his last trick. His 4th wall breaking is so huge that he can now even break the Mercy, Item, Act (probably) and Fight buttons. He can shoot lasers from his hands and head and his bones are very strong. He can even destroy the bullet board. Omega Flowey, on the other hand, can shoot a laser from his mouth and shoot nukes, pellets and triplets, so in my opinion Ultra Sans will probably win. But wait! Omega Flowey, absorbing all of the six SOULs, can use the abilities of the six humans, and can reload and make new save files, which Ultra Sans can’t do. He can also make weird hand-vines and shoot stars. He has flamethrowers that emerge from his... armpits? Yeah, that’s right. I’m not kidding.
However, this is most likely only the tip of their Iceberg of abilities. They’d most likely be on equal terms if not for one factor. The original host for the SOULs. Who’s to say that Sans’ would lose his Karmic Retribution and poison effect. In fact, the chances are that this would be enhanced by his power. Sans, when compared to Flowey in their original forms, was also more powerful and it’s stated that Flowey was never able to complete a Genocide run because of The Skeleton.
Flowey On the other hand, possesses Determination, being stated as before, able to LOAD and SAVE. However, because of all the death, destruction and torment Flowey has purposefully inflicted on others, he’d be very much susceptible to Sans’ KR and Poison Ability, Skill And overall strength.
In Conclusion, the fight could go either way, but the odds are in Sans’ favour. How Flowey would most likely win, is that he’d learn Sans’ attacks.