I actually think it starts to decline about half way through series 7. Not that series 7 and series 8 and to a lesser extent series 9 and 10 aren't still great, but they're not as consistently great and you can start to see the cracks coming in some episodes and it's not quite the "best ever tv" material that series 2-the first half of series 7 are (with some exceptions of course).
I've never understood the peak being series 3-whenever opinion though. Like Neg, I think series 2 may actually be my favourite series, though possibly 3 may be better, 2 & 3 are definitely the "absolute peak" for me. Don't understand why series 2 gets written off by so many people. Series 1 was very sketchy and hit and miss, but by the time it gets to series 2 it's all there imo.
Season 2 is fantastic and I don't see how anyone can say it's less than that. The show isn't as funny as it would be in the following few seasons but it has a powerful heart that I don't think the series ever retains again on such a consistent basis. It's similar to season 1 in some ways, but it's a much more strongly fleshed out version of Season 1.
With the different showrunners, the show would take strong variations in direction, but Season 2 represents the best of an era that is invaluable to the show.
And also, it has Lisa's Substitute, which is one hell of a tear-jerker.
"You are Lisa Simpson."
And here I start to tear up again....