I said 5 to intentionally make it difficult.
I had to cut Homicide, The Simpsons, and St Elsewhere.....
I would definitely say that 1 awesome season isn't enough to make my list, but maybe someone else can watch that same awesome season 20 times without getting bored.
Not sure how much crappy later season should detract from the overall rating. If it just goes to hell and betrays everything it stood for, then I guess it drags everything down. If it just kinda goes limp, then not so much.
All for you to decide.
They're your rankings.