Are you talking about hypothetical meltdowns or historical meltdowns? I am guessing historical.
Atari 2600 was probably the worst of all time.
Sega leaving the console making industry was pretty bad. People don't seem to realize that as a third party they don't really care enough to perfect a game before it releases.
Rare going to microsoft is a close third. We have forever lost one of the greatest development teams of all time, and I don't think the outlook of their franchises is much better.
I'm somewhat torn about this next one. Sony pisses me off in a lot of ways but they did fix Nintendo's crummy attitude and they expanded the gaming industry many times over. I think they were necessary at first, but they've done horrible things in the last generation that has forever ruined countless franchises.