The Wire is pretty explicit about its criticisms of institutions under capitalism. It's showing how those institutions don't guard against opportunists and the corrupt, but actually encourage them. We see how good people don't make it very far while absolutely diabolical scumbags climb over each other to the top.

I think what seperates it from other shows with villain protagonists is Baltimore itself is the main character. It's also better with each re-watch. I think the first two times I watched it, I'd say season 1 > 4 > 3 > 5 > 2. The third time I'm leaning more 1 = 4 > 2 > 3 > 5.