Because there is no concensus beyond genre, it becomes far too subjective. There are people who just won't consider a sci-fi or fantasy movie an overall GOAT because it isn't realistic. So about the greatest concensus you'll ever reach is that it's a great movie in its genre, and can therefore be considered a greatest movie period by those who most appreciate that genre. I'm sure there are people who consider T2 the greatest movie of all time. There are many movies who can't claim anyone as a fan of that level. But it's not going to be many people's GoaT.