Subject pretty much says it all. What is the best looking nitrogen fixing tree?
I think Mesquite is the most common, but it is not a good looking tree! It is a smallish tree with brutal thorns and tough seed pods.
You have lots of options in the tropics. (Eg Accacia) Fewer in temperate regions.
Eastern Redbud is a legume, but there is some dispute as to whether it fixes nitrogen. Ditto for honeylocust. Both are quite attractive.
Catalpa speciosa fixes nitrogen. Sort of funny looking to me, but it has flowers and a tropical look. Tough native but has brittle wood.
Black Locust definitely fixes nitrogen and is attractive but is quite invasive.
Speckled Alder and black alder definitely a nitrogen fixer but are a bit scrubby.
Brazillian Ironwood also known as Leopard tree
Its a stunning tree.
Here is a link that might be useful: Leopard Tree