If "neutral ground" means that each can bring their full power to bear, Shuma Gorath dominates.
Dormammu at the peak of his power could take second, while the latter-day down-and-out D. would place last.
On Earth, Shuma-Gorath would loose as Shuma can only extrude a tiny portion of himself into our world without destroying it. Shuma lost to Crom on Earth in the Hyborean age, and I don't doubt that either Set or D could match that feat.