It's so subjective. I mean, "Classic Albums" are usually defined by sales and mass popularity that over time end up continuing to resonate with an audience. That's not entirely fair, but generally speaking, as the public understands the term "classic album," that's how it's thought of.
For us, music snobs, it can mean totally different things. I consider the Images/Awake records to be Dream Theater's "classic" period where they were at their pinnacle. So those records are "classics" to me. But I can easily understand why others would say something like SFAM or another record is.
It's almost pointless.