Season 5 is the most underappreciated season, but I've really started to appreciate it.
McNulty has been a character who believes the ends justify the means. In Season 1, he falsifies that Sidner (iirc) was on the roof, thus allowing the police to admit phone tapped evidence.
In Season 5, it shows how taking that mentality of by any means necessary can quickly spiral out of control.
There is just enough foolery to make it easily tier 2 for me: Francis Wolcott's story arc, Olyphant's acting, Seth Bullock's attempt at morale highground.
If folks aren't turned off by violence, profanity, and prostitution, it's definitely must watch tv.
Oz is still one of my favorite tv show, it has so many good characters and the huit clos format (closed doors) is really perfect for focusing on them