About season 2: I've said a few times I preferred season 4 to season 2, which doesn't mean I think the second one is crap, far from it, and it's not the plot that bothers me at all (I liked the stories on the dock, and Frank Sobotka is one of the strongest characters of the show), it's just that I think there are some flaws in its structure, like they don't exactly know where they're going with character development (from the major crimes unit I mean), and that's especially clear with McNulty's character who is used a bit weirdly throughout the season. The creators said it was a transitional season, and it has a unique feel to it I think with the docks, but it does also really feel like a transitional season. But, again, by no means is it poor, it's sensational television.
I'm almost towards the end of season 4, and it's just reinforced my belief that it's my favourite. It's absolutely flawless, the writing is incredibly precise and the structure of the episodes is more refined than it was in the previous seasons. The acting by the children is terrific, I'm actually really impressed with the character development of Duquan: he's hardly a central figure in the Lafayette boys, you see a lot of Michael, Namond and Randy, but he's more peripheral, and yet his character is really fleshed out with subtle little pieces, the way he looks at his peers, the way he reacts to Prezbo's kindness, etc. He's really great, I remember his story affecting me the most, and it's the same this time around. For all four of them, there's a sense of inevitability as to what's going to happen, cos of their education, the people that surround them, etc.
And the political side of the show they brought in with season 4 is really interesting, Carcetti is great. Season 5, I don't remember that well, I do remember being really unimpressed with the fake serial killer story, but I also remember liking several points about it: Carcetti's disillusionment as mayor, Bubs's attempts at social reinsertion, the continuation of the stories of the kids and the media part. I thought it was maybe the 'weakest' of the five, yes, but I still found it to be very good TV and wouln't be too harsh on it.