I've heard a lot of comparisons between City of God and Slumdog Millionaire. While there are some superficial similarities, City of God is far and away a better film.
In many ways, The Wire is a better point of comparison for City of God than Slumdog Millionaire. Like the Wire, the story of City of God does not revolve around a single character but around a host of characters inhabiting a slum outside of Rio de Jineiro. Structurally, the film is made up of a series of interconnecting stories detailing the growth of organized crime within the slum. Stick-ups lead to drug dealing which leads to conflict over territory which leads to an escalation of violence that nearly engulfs the whole community.
The gangs throughout City of God, just like those in The Wire, are composed primarily of children and teenagers. This isn't The Godfather where grown men gun each other down whilst wearing suits and ties. City of God, which frighteningly is based on actual events, features children committing heinous acts of violence for reasons that are unclear even to them.
What makes the film so strong is the frankness in which it presents this fact. While the film has some unique cinematography, there is a distinct lack of aestheticization of violence. Every bullet thats fired has consequences, random shots end up having grave consequences. Even Knockout Ned, a character whose motivation for violence is initially laudable ends up losing his moral center as the violence escalates. The fact that the film ends almost exactly as it begins underscores how pointless and inevitable the cycle of violence has become.
I haven't seen the follow-up City of Men or the TV series of the same name yet. But after rewatching City of God, I'm definitely going to check them out.