Depends on the point of view - from a specific school's pov, a conferences, or the overall, 30K' view, right?
Aside from MSU joining the Big Ten, I think modern realignment started in the 70s with the AZ schools joining the Pac. I'd call that a win for everyone, as ASU went from a modern Boise-type program to in with the big-boys and that $$$. Arizona was along for the ride, but helped with basketball, eventually. And the PAC got to grow, but kept its haves winning the conference more often than not.
It was only in the last couple of years that I learned GA Tech joined the ACC in the mid-80s. That wound up really helping them in 1990, allowing them a weaker bowl opponent (no offense, UNL) that year than if they'd still been independent. They have a NC...which means the ACC has that NC, too. Win-win.
That takes us into the 90s, when all hell broke loose (thanks, SEC).
I think it's safe to say the original Big East, while not a top dog, was fine, in terms of strength (as long as Miami was Miami). But once they got poached, and due to contracts, they were still involved in the big-boy bowls, I believe it's a 3-year stretch of a compromised Big East being treated like a big-boy conference......that was the thud or the worst move.
In terms of a conference inviting a team and it being a mistake.....got to go Rutgers here. Yes, I'm a proponent of adding chum along with the shiny, sexy programs.....but Rutgers is below that. It's an anchor, in a bad way. I don't care if the low viewership in a huge place makes financial sense.....adding Rutgers was a bad call.
Biggest miss? A Big East with Miami, Penn St, and ND would still exist today, imo. It could have happened. Maybe it would have kept things static for a longer time, idk. It would have been cool to watch.
