Who is the hardest boss on undertale?

Answer 1

omega flowey

Answer 2

That would be debatable between Undyne the Undying (Genocide) or Sans (Genocide). Undyne the Undying's attacks are harder to predict. Sans is hard to beat the first time, but relatively easier afterwards because his attacks are always the same, and easier to memorize because of how many tries it usually takes players to defeat him.