What is considered the most valuable LEGO set? Which Lego set is considered to be the most valuable set to own?

I'm not talking about the most expensive set, and clearly it has to be out of production already, but when trying to collect complete sets from LEGO history, there must be one or a group of sets that are prized possessions for collectors.

Which are they?
I believe it's the Ultimate Collector's Edition Millennium Falcon (10179).

With a $500 MSRP, it was already one of the most expensive sets available when it launched in 2007. In 2012 it is rare to see it for less than $1500 used but intact, and closer to $2000 if you want it new. There may be some smaller sets that have a greater percentage gain (a set that was $2 going to $20, for example), but I suspect at $2000 and a $1000-1500 difference, this is the largest magnitude.