Based on spell damage? None?
Best MP regen in the game is Essence of Carnage guest, toggled to that mode. Heals hugely based on your *weapon* attack. Charisma boosts it
The Siphon spell can be good if it's boosted from generalists robes.
All fairy godmother pets
Meteiod/cometoid jelly pet is good
Theres a handful of other pets
If you're asking due to Lich Form, all HP heals convert to MP heals.
Best HP heal guest might be Nightbane guest, from nightbane claw staff.
Maybe Twilly? The twilly wand is another HP heal source