I hate to be cliché, because I think most people consider this season the best, but it really is an excellent season. Maybe it's because most of the characters are already established solidly, or that one develops a better connection with the kids in the season because of their initial innocence, in contrast with the rampant corruption around them.
Be what it may, this was the season that affected me the most, and I think it was mostly because of the kids. Not only are the performances of all four of them excellent, but their situations feel real and one can't help but feel pity for them as they try to escape poverty, drugs, jail, abuse, or death.
Another thing I liked is the way they develop the Marlo Stanfield plot. At one point, I was afraid that the departure of Stringer Bell would leave a hole in the show, but after a few episodes I was already going "Stringer who?". Marlo and his people are so ruthless AND clever that I felt like screaming at them cops just to make things right so they could capture him. And every time he sneaked away, it's like "damn!"
Finally, the further exploration of politics and its machinery, was something I particularly "enjoyed" because I'm very much into political debates, and it cemented my lack of hope towards the government. It's sad, but a lot of times I went "they got that shi
t right", as they showed politicians and high-rank officers scheming and manipulating whatever they needed to curry favors and public acceptance. Ugh...
Least Favorite Season
Again, I don't wanna be clichéd, because I think most people consider this the "worst" season of the show (whatever that means!)... but I do think it was the weakest, or least good. Which means, it still was pretty great, but the rest of the seasons were better.
The thing is that, I feel like it's my least favorite for the "not so usual" reasons. First off, I thought the most important and well-developed subplot was that of Frank Sobotka and co. as he tries to keep the Baltimore port afloat. Most of the performances involved were great and I felt it was a good change of environment.
What I liked less is the way they handled the lead police characters, especially McNulty. I pretty much hated him this season, and how the writers wanted to show him as the most reckless, drunken, stupidest man ever. IMO, it felt forced the way they had him jumping women all the time and whatnot. They could've achieved the same results more subtly.
Also, generally speaking, I wasn't that crazy about the way they handled the Major Crimes Unit. I don't know why, but everything felt like an excuse to put all the same characters together to do another wiretap. As opposed to future seasons, that flowed more nicely and neatly, this one felt awkward at first.
Like I said, the season is far from bad or awful, but I still don't think it worked as well as other seasons.
As for the other three seasons, I really wouldn't know how to rank them. Part of me wants to say that Season 1 is probably another weak one, but the thing is that I loved it. I think the top and bottom are very clear, but these other three? I wouldn't know.