Ashur, definitely. One of the gems of Fallout 3. Ashur is a leader, knows his limits, and isn't a delusional idiot who gets kicks for oppressing minorities. Pitt will definitely become NCR-like powerhouse on the East Coast in the long run. Truly, the only faction I appreciate besides the Enclave.
I never really liked Caesar's Legion. It's explained well and all but I can't help but to think it's just too over-the-top for the setting. They are organized but they have serious fundamental flaws ranging from class society to having the nation as personal property of its founder (Caesar constantly refers to the Legion as "his" Legion, and naming a nation after yourself will lead to problems after you die). Really, the only good side in the Legion is that they like killing, includes raiders. Other than that, they are the worst thing to walk the Wasteland since Master's Army.
Legion isn't a long term society. It lives on blood and it will run out sooner or later. We only have Caesar's word that things will become better after Vegas is his. But I wonder how many times he has said that? Caesar is the guy who wages total war even he will die of cancer unless the Courier saves his sorry ass.