1. Marlo - Marlo was the most bloodthirsty and Spartan of the three. He didn't need family. He didn't need sophistication. He was as smart as any of the three, and able to compete mentally and militarily with all. He trusted no soul, not even Chris really. He'd turn on anybody, even his own troops if he felt they were weak or anything other than 1000% loyal.
However, Marlo was soulless in a way--his life was gray and brutal, he was like some kind of slim gray shark that lived to swim and kill. And emotionally, he was a fuckin' brat--couldn't take getting whupped in cards, couldn't take getting clowned by his enemies, he was petulant and petty and didn't respect the mandate to move quietly among civilians as much as possible. The Wire never solves this for the viewer but its my understanding that he felt more trapped in his suit playing the String role for the crackers than Avon felt locked up in Jessup, and eventually returns to the block and gets killed or jailed.
2. Avon - Was as smart as String and violent as Marlo, but unlike String he believed in honor and code--there's a WAY to being gangster that is sacred ("You know the difference between me and you? I bleed RED, you bleed GREEN.") This is because Avon came from a famous family of gangsters that ran Westside for most of the 20th century. There was a nobility in Avon all the others lacked, and even as Marlo took over after Avon's fall (which he played little part in), he couldn't get muthafuckas to shut up about the Barksdales, which annoyed him constantly. Even when he tried to circumvent the Co-Op, Avon tripped up his hustle and extorted 100 stacks out of him for the privilege of even SPEAKING to Sergei ("Boris...why always Boris?"). Avon believed in family, he built up the civilian community more than any of the three. He was the kind of crime boss the Law was willing to tolerate due to keeping the streets stable, with bodies and waves at a minimum.
3. String - I personally loved String's character the most because he saw the move Jewish, Irish and Italian gangsters made to legimitize and move the fuck off the block to bigger and better avenues of power. He studied military, economic and political philosophy and history and principles. He dug on Feng Shui and real estate and technology and black power. But String never really saw himself as 'hood in the way Avon, Marlo, Bodie, Mike, etc. did. He was coldhearted and selfish and willing to throw anyone under the bus if he felt it was the logical play in that situation. As much as I liked String, he did so much foul shit that after the shock of seeing him get dropped wore off, I had to admit he deserved it.
Me, I say Avon was ultimately the man over all the other gangsters, he just lost perspective after getting out of Jessup the first time and tried too hard to re-establish his rep. But it was clear that if String had not given Avon up to Major Crimes, Marlo was a fuckin' dead man.
