Oldest? Probably Levistus.
Most loyal? Probably some low level prime.
Oldest and Most Loyal? Probably Bel or Glasya.
Edit: I felt the need to clarify. Loyalty does not run strong in the Hells, obediance, duty and fear is what keeps the Hells running smoothly. They are lawful, but do not mistake taking orders as being loyal. Any duke would gladly turn on Asmodeus if the opportunity presented itself. But taking on Asmodeus is like fighting Kord bare handed, if Kord had nukes.