Must See TV, on NBC
This was more of a slogan than an actual block, but still it was kind of an important part of the 90s, so I thought I should at least mention it. Some websites may tell you that the slogan had its roots in the early 80s, but I don't think that's worth worrying about. According to Wikipedia, the slogan was first used in August 1993, a few weeks prior to the start of the new fall season. (That sounds about right to me; I couldn't have specified a year, let alone a month, but I was sure it must have been sometime in the early 90s.) So, that's where I shall begin. The first block to bear the name "Must See TV" was Thursday nights (from 8 to 10pm), and included Mad About You (1992-1999), Wings (1992-1997), and Seinfeld (1989-1998; the series would hold the 9pm slot for the rest of its run). Starting in September 1993, the lineup would also include Frasier (1993-2004).
For the second season of Must See TV, Mad About You continued at 8, Wings was replaced at 8:30 by Friends (1994-2004), Seinfeld continued at 9, and Frasier was replaced at 9:30 by "Madman of the People" (1994-1995). (In spring 1995, Friends replaced Madman, and Hope & Gloria (1995-1996) took Friends' former timeslot.) Also in the 94 season, Wings and Frasier moved to Tuesday nights, which would also include The John Larroquette Show (1993-1996). At that time, NBC started using the "Must See TV" slogan to refer to Tuesdays as well as Thursdays. And I guess they'd later use it for other weeknights, but for the most part it refers to Thursdays nights, which is what most people remember it for. So I think from this point on, I'll limit myself to only mentioning Thursday night programming, even if that may technically mean I'm excluding some shows that aired under the Must See TV banner. I also feel like mentioning that, while many of the Must See TV shows were big hits, and may have contributed a certain number of viewers to other shows in the lineup (presumably because some people were too lazy to push a button on their remote to change the channel), there were any number of shows that weren't hits, and didn't last long. In fact, when I think of "Must See TV," the show that mainly comes to mind for me is "Friends," and to a lesser extent "Seinfeld" (the latter of which was the main reason the slogan was coined in the first place).
Mad About You moved to Sundays, and was replaced on Thursdays at 8 by Friends, which moved up from its prior 9:30 slot to anchor the block (it would continue to hold the 8pm slot until it ended in 2004). Hope & Gloria was replaced by The Single Guy (1995-1997), which itself was replaced in spring 1996 by Boston Common (1996-1997). Seinfeld continued at 9. The 9:30 timeslot was now filled by Caroline in the City (1995-1999).
The Single Guy reclaimed its former timeslot from Boston Common, but was replaced in winter by Suddenly Susan (1996-2000). The 9:30 slot was filled at different points by Suddenly Susan, The Naked Truth, and Fired Up.
The 8:30 slot went to "Union Square" (1997-1998), but that was replaced in winter by Just Shoot Me! (1997-2003). The 9:30 slot was taken by "Veronica's Closet" (1997-2000).
Just Shoot Me! was replaced at 8:30 by "Jesse" (1998-2000), which itself was replaced in spring 1999 by "Will & Grace" (1998-2006). With Seinfeld now over, Frasier moved back to Thursdays to take over its 9pm slot. Veronica's Closet continued at 9:30.
Will & Grace was replaced at 8:30 by Jesse, which was replaced in spring 2000 by "Daddio" (ended October 2000). Frasier continued at 9. Veronica's Closet was replaced at 9:30 by Stark Raving Mad (1999-2000), which was replaced in spring 2000 by "Battery Park" (ended April 2000).
Daddio was replaced at 8:30 by "Cursed" (2000-2001). Will & Grace replaced Frasier at 9. Just Shoot Me! replaced Battery Park at 9:30.
Cursed was replaced at 8:30 by "Inside Schwartz" (2001-02), which was replaced in winter 2002 by "Leap of Faith" (ended April 2002), which was replaced in spring 2002 by Friends (which was still airing at 8). Will & Grace and Just Shoot Me! continued at 9 and 9:30.
"Scrubs" (2001-2010) took over the 8:30 slot. Will & Grace continued at 9. Just Shoot Me! was replaced at 9:30 by "Good Morning, Miami" (2002-2003).
The 8:30 slot alternated in fall between Scrubs and Friends (the latter still aired at 8), and was replaced in winter by Will & Grace (which still aired at 9), and in spring it was Friends. The 9:30 slot alternated in fall between Scrubs and "Coupling" (Sept-Oct 2003). In winter, "The Apprentice" (2004-present) aired from 9-10, being replaced in spring by Will & Grace at 9 and Scrubs at 9:30.
With Friends now over, it was replaced at 8 by its spin-off, Joey (2004-2006). Will & Grace moved from 9 to 8:30. The 9-10 slot was now taken by The Apprentice.
To be honest, I don't remember for sure how long the whole "Must See TV" thing lasted, but this is the last season that Wikipedia lists it in its grid. For all I know, it could have ended a few years before this, but I might as well go along with this as the last season. So... Joey continued at 8, but was replaced in winter by Will & Grace. The latter also continued at 8:30 until being replaced in winter by "Four Kings" (Jan-March 2006), which was replaced in spring by "My Name is Earl" (2005-2009). The 9-10 slot was held by The Apprentice in fall, but in winter was replaced at 9 by "My Name is Earl" and at 9:30 by "The Office" (2005-2013). Yeah... I guess I might vaguely remember The Office being called Must See TV, but since I only watched the first episode, I can't remember for sure. Whatever, I guess this is a good time to say the block/slogan ended.
Wikipedia says "Must See TV" was replaced in fall 2006 by "Comedy Night Done Right," which lasted through the 2012-13 season. I suppose I must have heard that slogan, but it doesn't ring any particularly loud bells, so it's not something I'd care about calling a block.