30 Rock.
The first couple episodes were nothing special at all. I watched them and didn’t like them and abandoned the show altogether.
I came back to it and found that the first season was okay- kind of fun, but as I was watching it, I still didn’t get why everyone seemed to love this show so much.
I think it hit its stride around the second half of the second season and got better from there, for the most part.
NZ soap Shortland St was really dire for at least the first year (they brought out a special edition DVD of the first couple of episodes. I own a copy so I know how bad it was) It wasn’t only the bad acting but the production values were really bad.
Its in decline again now. I wish they would cancel it & try something new.
The first season of **The Simpsons **was truly craptacular. By seaon 3, it was on its way to being one of the best shows in the history of TV.
I profoundly disagree. The fourth season had too much TNG/TOS continuity porn, & the finale tried to make ENT about TNG (which had lost me with its first two crappy seasons).
I liked the Mirror Universe eps, though.
Along with the first two seasons, I also like to pretend that the finale doesn’t exist.
Rhoda was one show that actually got better and better each season after the 2nd. After Joe and Rhoda married it was dull, but once Rhoda got divorced, each season the show improved. It picked up Ann Meara and Ron Silver as regulars and it did improve.
Unfortunately thin-together-Rhoda was no match for fat-funny Rhoda and the Mary Tyler Moore Show went into reruns viewers could now see the contrast and that spelled the quick end of Rhoda. But it actually did improve each year after Rhoda’s divorce
Unfortunately, they had to replace Coach after Nicholas Colasano’s death during season 3, but I agree that the show improved with the additions of Frasier and Rebecca. Woody was fine, but that was more of a lateral change.
30 Rock is in its 4th season now, so unless this season is better than the preceding 3, it should be disqualified.
Nobody held up much hope for The Big Bang Theory and the first season was mediocre at best, It was only when Jim Parsons broke through as Sheldon Cooper that it got great!
I’ve got to disagree… I still don’t think it’s a very good show, and what’s more annoying is I keep getting people saying “Hey, you’re a bit of a nerd, you must think it’s the greatest show on TV!” and being really surprised when I say “No, actually, I don’t particularly enjoy it.”
Personally, I agree South Park took a couple of series to get going- it was amusing and funny from the start, but then it got really good and become very clever indeed. Ditto The Simpsons.
Also, the second and subsequent series of Blackadder are far better (and funnier) than the first one. 
Another vote for Mary Tyler Moore, but for a different reason. The first two seasons set specific parameters on what everyone could do and still be in character. Season three and four, they start breaking the rules–Rhoda slims down and competes with Mary for men. Lou gets a divorce and starts dating again. Mary shows an ability to get cutthroat. Sue Anne poaches Phyllis’s husband. Mary contemplates the disadvantages of maintaining a friendship with Rhoda. Murray admits to himself that he’s profoundly disappointed with his life. It all builds off of what was established earlier, but they have more freedom to break new ground.
The Big Bang Theory is only in season 3.
Heroes is the poster child for being great the first season and sucking thereafter.
Newhart (Bob Newhart’s 1980s CBS show) greatly improved when they replaced Kirk (or was it Kurt?), the liar who managed the cafe next door who was in love with the Louden’s maid, and the maid, with Larry, Larry & Darryl running the cafe and with Stephanie the heiress as their new maid, and added Michael the TV show producer as her love interest.
“Everybody Loves Raymond” improved greatly with every season, although I hate, hate, hated the series finale. I don’t particularly remember what I thought of the first season when it originally aired but when I see the reruns now I wonder how it made it to a second season. Good god, Patricia Heaton was abominable. That atrocious, frumpy hairdo and wardrobe (to be fair, I think she may have been pregnant so the frumpdrobe may get a pass)make me want to crawl through my televison set and bitch slap her. I know a lot of people here hate this show specifically because they find her character a shrill bitch but I think her comedic timing soared as the show went on and I really enjoy watching reruns from the later seasons.
The early Doctor Who incarnations were good for their time, but the show really hit its stride between “The Three Doctors” and “The Five Doctors.”
Ever since they announced a definite “three more seasons” end point to Lost after the end of season 3, they’ve really been pulling it together into one great big ball of awesome. No more “the origin… of Jack’s… TATTOOS” water treading, and while they might not answer every little thing well enough to please every last nitpicker, seasons 4-6 have just been full of awesome and win.
Barney Miller–I just watched the whole run and excluding a slight overreliance on Inspector Luger and making Wojo retarded*, the show improved hugely. They got rid of Barney’s whiney wife (mostly). They got rid of Fish (who was a one-joke character that they ruined when we saw that Bernice was essentially a sweet old lady like Edith from ALL IN THE FAMILY), they got rid of Chano, Jack Soo died (which was a huge shame–they never knew what to do with the character and Soo was such a solid actor).
The cases got weirder, Harris bloomed into a brilliant character, Deitrich was exactly the ingredient needed to make the cast click (and against tradition, Harris was upset that he was displaced as “The smart one”) Hell, even Barney stopped being sweet, loveable, wise old Barney. By the last season, Barney realized that he’d been in that (apparently dead-end) job in that office for 10+ years and was suffering severe burn-out. Which is pretty damned dark if you think about it. And they still managed to make it laugh-out-loud funny AND avoided “very special episodes”
*In the last two, three seasons, his IQ dropped to like 70. In the early episodes, he was just a redneck jackass who wasn’t really bright. By the end, they tried to turn him into kind of a proto Bull (from Night Court).
Angel. The last two seasons are the best.
And I agree with The Shield.
B5 didn’t end at it’s peak like DS9, but its fourth season is generally considered the best. Actually, I think that arc shows are going to be the winners here - as the pacing picks up, the show gets more gripping, and there’s less extraneous stuff.
Another candidate is Blackadder - the fourth series was my personal favorite, at least. Blackadder is so much better when he’s out of power.
Season 4 was Angel’s low point. The show peaked with Season 3, then re-peaked with Season 5.