I’m not married to this position. I lived in a college town when Twin Peaks was on, so probably more people were watching that show in that town than ever watched Dallas. I don’t have my fingers on the pulse of the country, though. Whatever.
Yeah, I think that’s about right. Twin Peaks started out with a lot of critical fanfare and had an early cult following but it was just too weird for most viewers, and even the critics ended up largely abandoning it as too opaque and aimless. It didn’t help that David Lynch himself had no real interest in ‘solving’ the murder of Laura Palmer; he was looking to make an surrealistic horror/fantasy drama that was an exploration of the American Gothic themes of social disconnectedness, petty corruption, and sexual deviance combined with Lynch’s trademark idiosyncratic absurdist humor. Framing it as murder mystery allowed Lynch and Frost to make an elevator pitch out of it but Lynch apparently lost interest pretty early on when it was clear that the limitations of network television weren’t going to let him go off into Blue Velvet territory. The show’s audience dwindled even during the first series, and the fell off a cliff as the central mystery wasn’t revealed until the middle of the second series, by which time ratings were so poor it was never given consideration for renewal.
Dallas, on the other hand, was a pretty standard issue prime time soap opera elevated by the utter ruthlessness and amorality of several of the characters and particularly the aforementioned J.R. Ewing (the sociopathy of which would not be equalled until the short-lived 1996 Profit). It was pretty easy to follow as the plotting primarily consisted of the standard tropes of people sleeping around with each other, seeking to undermine each others success in business and love, et cetera. The “Who Shot J.R.?” plotline was notable for the unexpected grievous wounding of a major character but as a program it was already a highly viewed show with a popular following. I would agree that people didn’t go to “gatherings” to watch it, but then, such initial enthusiasm didn’t sustain Twin Peaks for even the season-and-a-half of the initial run while Dallas was on for twelve seasons, all but the last couple as a consistently top ranked show in its time slot.
FWIW, I personally think Twin Peaks was a more interesting show and arguably had more influence on the direction of modern atmospheric television programming with metaphorical themes and offbeat humor–certainly shows like The Sopranos, Mad Men, and Breaking Bad all owe acknowledgement to Twin Peaks for breaking the standard mold of what could be done in the medium of television, while Dallas later became kind of the punchline for the “It was all a dream” trope for reversing a prominent character death when Patrick Duffy decided to return–but it is pretty incontrovertible that in their respective times, Dallas had far more consistent viewership and the “Who Killed J.R.?” plotline generated an ample year’s worth of (mostly pointless) discussion and theorizing, while the viewing public mostly gave up on Twin Peaks before its first abbreviated season was even completed.
Stranger
Guys I think we forgot the most obvious one happening right now on this board:
Wandavision
Though, the OP was asking about shows that accomplished that level of conversation prior to Lost and Battlestar Galactica – so, in other words, prior to around 2004.
Heroes started in 2006, while Lost started in 2004. And as for “Who Shot J.R.?”, we went to London that summer, and seemingly everyone we met asked us about it. I think they were a few episodes behind.
Ah true. Missed that.
Yeah, I missed that the lead sentence was a carryover of the title, I thought it was just an example the OP was throwing out there. 
We had 6 or 7 people watching at my place. I furnished the coffee, the others took turns bringing pie. Those were the only watching parties I ever had until Game of Thrones, but we didn’t have accessories for those. 
Cherry pie. Damn fine cherry pie.
We had watch parties for Dallas, for what was basically a soap opera turned up to 11, it had a viewership that was both large and fanatic.
MASH* had a lot of water-cooler theorizing for its finale. We knew the episode would be TV movie-length, and we obviously knew the war would end. But there was a lot of speculation that one of the main characters would die, that Hawkeye and Margaret would end up together, that sort of thing.
I imagine the majority here is far too young to remember “Peyton Place (1964-69),” but it was huge! It was the first bona-fide prime time soap opera, and launched the careers of Mia Farrow and Ryan O’Neal. It aired in half-hour episodes twice a week, and it was all anyone could talk about, especially the first two season when Mia Farrow was on it!
Good call. It was before my time but it was enough of a pop culture phenomenon to be referenced on Mad Men, and its plotlines were certainly pretty scandalous for its time.
Stranger
Luke and Laura’s wedding on General Hospital was in 1981, but it was the culmination of a few years of edgy storylines that ending up capturing a huge audience, far outside the normal soap opera crowd.
I seem to recall theories about what’s going on in The Prisoner posted here as early as 2001. I’m willing to bet it was discussed even before that.
I know I quit then. Didn’t watch the revival, didn’t watch Lost, either, because of Twin Peaks.
And I’m still waiting to know what’s up with the fish in the percolator!
I re-watched Twin Peaks around the time of the revival, and I was struck by how much of a primetime soap the show actually was. Sure, there was some weird stuff, but there was a lot more boring business intrigue, soapy romantic shenanigans, and the least convincing “teenagers” in television history. It was much closer to Dynasty than it was to today’s high-concept TV shows.
Your last sentence can be summed up this way: It was.