Season 4
Let me start by saying that the season 3 finale was about the best season finale I've ever seen. Additionally, the characters throughout the first 3 seasons had a rare magical quality of being likeable despite being tragically flawed. That balance has shifted significantly in season 4. Drake, rather than being the sympathetic thug used constantly by Reid as his personal battering ram is now more of an aloof boor, particularly with regard to his relationship with Reid. Long Susan is now off the rails despicable. Jackson has come off as rather wimpy and p-whipped. Rose, who has always been an up and down character, is now almost constantly down/loathsome. And Mathilda is neither here nor there.
I personally think that this show would have been better if it had simply ended with season 3. It would have gone out on a very high note, rather than descending as it has in season 4.
"Lettin' the cat outta the bag is a whole lot easier 'n puttin' it back in." -- Will Rogers