Actually, there are numerous cities in America that have had murder-rates rivaling the most dangerous non-warzone areas in the world... New Orleans, D.C., Detroit just to name a few.... At their worst, those cities were damn horrible... Corrupt police, corrupt politicians, high murder-rates, all of that...Everyone likes to think their city is the hardest, but the worse ghetto even in 1980s America or anywhere else in the 'First World' isn't on the same dangerous levels as Brazilian slums by a long shot (you should be happy about that anyway). That's just an undeniable fact. The police there many times ADD to the problems more than any corrupt USA police officer can do in the hood. Anyway, this thread isn't about that....
To answer my own question.....I have to go with Lil Ze obviously. Marlo was more intelligent in a lot of ways (even though he didn't know the basics of having a bank account) but let's not act like he wasn't insecure too. The only reason he pulled that shyt with the security guard was because he wanted to feel like a man again after losing his money in the poker game and getting sonned by those older dudes. Everything with him wasn't business. He even 'joked' with Chris at one time and said he would send him after those dudes if he ever got bored. I could also see him pulling the same shyt in a club Lil Ze did if he was jealous of another dude getting girls and he couldn't (that was so foul). Both dudes committed or ordered murders that were totally pointless, but overall Lil Ze takes the crown. The hand or foot scene had me in shock for like 5 minutes afterwards when I first saw it because I never saw something like that involving kids on screen before.
Also to add to the thread, overall if you wanna compare both drug crews, Lil Ze's crew was more ruthless and if they battled Marlo's crew, they would have crushed the buildings.
As for Kenard, he would have fit in well with those kids. The reason he got scared when he shot Omar was obviously because that was his first kill. After the second or third time it would have been. Plus those kids in COG also got slapped around a bit, but what can you do against a dude who is like 8 years older than you and physically stronger? Lil Ze himself got slapped up by that cat in the Tender Trio when he was younger.
I could see Knockout Ned and Omar forming an alliance because of the similarities and common goals.
A fantasy 'The Wire meets City of God' movie would be the shyt.
But to answer the thread... Lil Ze just because he was definitely written as a more ruthless character than Marlo was
Like the Mexican/Colombian cartels leaving 100's of dead, sawed off limbs packed into a van dripping with blood and brain matter as a sign for people not to fukk with them 
or a dude getting beheaded with a chain saw, watching his friend go first and his friend's blood splattering over him 
then getting his own head sawed off til the point that it was only hanging on by a few nerve endings 
look was just.....
We gonna act like people like Marlo and Kenard weren't just as ruthless as those kids in City of God? Put them all in the same position, same scenario, and you're getting the same results.