Well, as much as Manos was a Defining Moment for me in MST, for sheer comedy value it's tough to beat a, say, "Space Mutiny" (hey, Big McLargeHuge) or "Cave Dwellers." But I can't sit through something like "The Unearthly" without nodding off, unlike "Manos" which is consistently entertaining. Your milage may vary, I s'pose.
Don't make me bring back the "Rocket Man" avatar. I like Leonard Cohen where he is.