Television is enjoying a renaissance these days as the creative people in show business realize that season-long arcs can be a more compelling way to build a story than two hours in the cinema.
The shows also tend to generate a lot of controversy and debate about their high and low points. We pulled data from IMDB user review scores for some of the most compelling, popular, or immortal shows of the the past couple years to try to find when they peaked according to viewers.
On IMDB, registered users are able to give numerical reviews, 0-10, of television episodes. On the following charts, each episode is represented by a diamond. The x-axis is the seasons of the show. The y-axis is the score of each episode.
It's remarkable how one season can change the trajectory of an entire series.
We added a trend-line to the charts to track how the average ratings for the seasons changed over time.
Here we go:

"The Sopranos," which is widely credited with kicking off the small-screen renaissance, had outstanding odd-numbered seasons and slightly less-so, but still-outstanding even-numbered seasons. (In this chart, Season "6" refers to 6a and Season "7" refers to the final nine, 6b)

IMDB television scores
Walter Hickey/BI, data via IMDB

"South Park" on Comedy Central got its best reviews in its sixth and seventh seasons but remains an animation powerhouse.

IMDB television scores
Walter Hickey/BI, data via IMDB

We can just admit it folks. It's okay. The fourth season of "Arrested Development" kind of sucked compared to the masterpiece that came before. Now the healing can begin.

IMDB television scores
Walter Hickey/BI, data via IMDB

"It's Always Sunny In Philadelphia" won our hearts with a bold and unprecedented first season, but according to IMDB has become more erratic since.

IMDB television scores
Walter Hickey/BI, data via IMDB

After a rough first three season, "Seinfeld" hit outstanding quality for six seasons running. The eighth season is Peak "Seinfeld." 

IMDB television scores
Walter Hickey/BI, data via IMDB

"Breaking Bad" will not stop getting better. It's remarkable.

IMDB television scores
Walter Hickey/BI, data via IMDB

"The Simpsons," the journeyman of the group, peaked in season six and really started to decline in season nine, as thousands of loud, drunken arguments across America have by and large confirmed. Still, it's held steady for over a decade, which is nothing to sniff at.

IMDB television scores
Walter Hickey/BI, data via IMDB

"Battlestar Galactica" had a rocky third season but finished strong. Also, ardent fans of the series can actually see the reviled "Black Market" episode — in season 2 — from this chart alone. Just abominable.

IMDB television scores
Walter Hickey/BI, data via IMDB

"Dexter" has had a rocky ride, but the second through fourth seasons are made up of some of the best stuff they'd ever done.

IMDB television scores
Walter Hickey/BI, data via IMDB

"Family Guy" has seen a bit of a dip in quality recently according to IMDB, but the show has pulled off almost a dozen seasons of solid performance, which is an achievement to say the least.

IMDB television scores
Walter Hickey/BI, data via IMDB

"Friends" was a consistent performer for years and peaked right at the end.

IMDB television scores
Walter Hickey/BI, data via IMDB

"Mad Men" peaked in Season 4 and you know it.

IMDB television scores
Walter Hickey/BI, data via IMDB

"Futurama" has come back from the dead with remarkable regularity, so the show's sustained performance is stunning. Still, budget cuts, years off the air and a writer exodus hollowed out the new product on Comedy Central.

IMDB television scores
Walter Hickey/BI, data via IMDB

You may be shocked that "Grey's Anatomy" is still on the air, let alone doing some of the best work they've ever done.

IMDB television scores
Walter Hickey/BI, data via IMDB

This chart does not do justice to the roller coaster that was watching "Lost" in real time.

IMDB television scores
Walter Hickey/BI, data via IMDB

"The Office" got pretty unbearable after season 5. However, it's widely acknowledged that they pulled off one hell of a finish.

IMDB television scores
Walter Hickey/BI, data via IMDB

HBO's "Six Feet Under" started strong, lagged in the middle, but really pulled it off in the final season.

IMDB television scores
Walter Hickey/BI, data via IMDB

"The West Wing" was awesome, and then Sorkin left, so it stopped being quite as awesome.

IMDB television scores
Walter Hickey/BI, data via IMDB

The third and fourth seasons of "The Wire" were absolutely its strongest. Season two's "Longshoremen" and season five's "Baltimore Sun" subplots were considerably weaker stretches.

IMDB television scores
Walter Hickey/BI, data via IMDB

UPDATE: By request, here's "24."

television reviews
Walter Hickey / BI, Data from IMDB

And here is "Saturday Night Live." SNL has had an excellent track record over the past couple of years following a particularly rough patch.

SNL rating
Walter Hickey / BI, data from IMDB
And by Reddit request, Weeds. Looks like a clever concept will only take you so far.
Weeds
Walter Hickey / BI, Data from IMDB