Technically, ALL drivers have have the same COR on their faces and head sizes because of USGA and R&A restrictions BUT for some reason, I keep seeing the 2016 M2 still being talked about even after its discontinuation. I'm not saying it's the longest driver made in the last 5 years but it seems to be a little more consistently longer than the others (just a little).
The first gen M series was the first time Taylormade had really lived up to the hype they put out around it since the original RBZ. The M1 and M2 put out really solid numbers for most everyone that tried it, the composite top made it standout in a crowd, and I remember Jason Day putting it in his bag right away and absolutely smashing with it (he was also playing out of his mind at the time). There are also those rumors that float around here about the 8.5* heads being too hot but technically legal, which meant pros were bagging the M2 long past when they would normally switch. This combo of tour presence, excellent numbers, and bag appeal is what has driven it's current cult status. It also helps that Taylormade's current offerings seem to be more centered around marketing and gimmicks (face screws, really lol) than outright performance like the clean, no nonsense 2016 M2 provided.