Do you mean 1)Diplomatic wins using the UN or 2)Diplomatic wins with the AP?
If you mean 1) then financial helps with research, obviously and industrious helps with the 2 key wonders - the UN and Cristo Redentor. With the latter you have spiritual's benefits and can switch civics/religions whenever. Civs with mysticism are very nice but spreading someone else's religion is also doable; its just you dont get the money. The natural choice therefore is therefore Huayna Capac.
Bonafide's point is a good one and as you might not need the UN wonder speeded up (because no-one else has Mass Media) Mansa Musa is also good. Edit: actually Mansa is better; spiritual speeds up CR (which is still nice even if you are spiritual) and speeding up the build of the UN is far from game-breaking.
If you mean 2) then founding an early religion and using it as the AP one is BAD because you want as few foreign cities as possible with the AP religion so you can vote yourself in. It is best to bulb theology and found christianity.
Spiritual is much more helpful here because you can change religions/civics a lot to get open borders and bribe people out of theocracy (you can use spies as well but that is something else to build) Industrious also helps to build the AP is it has no speed-up production resource and can be quite expensive.
Therefore the best leader is Ramesses II. With the best two traits, a UB that can get you a GP and a UU that can take out a rival early he is far and away the most suited.