Kell would defeat Kurgan easy
After watching the entire series, and all the theatrical films (except "The Source" because that piece of garbage doesn't exist to me) I am confident that the baddest of the bad is Jacob Kell. Kurgan was a monster and a powerhouse, but Kell would have taken him out easy. Kell was fast and precise, while Kurgan left himself open by taking huge monster swings with his sword. Kell beats him in no time.
But otherwise, after watching everything, I can now conclude without a doubt in my mind that Jacob Kell is the baddest villain in the Highlander universe.