Hands-down, the best LEGO set ever made is 497 Galaxy Explorer. I had one of these as a kid and must have played the crap out of it, along with its smaller brother 918 Space Transport. About 5 years ago I re-discovered it in my Mum’s house when I found the instructions and enough bits to cobble it together in the attic (albeit a lot of the bits must be from the Space Transport; and I didn’t have enough bits to make the landing pad and little tower thing).
In the video on this page you can see a glimpse of the LEGO vault in Billund where they keep pristine copies of every set ever made. There’s a bit in the video where they get out the Galaxy Explorer and flip open the lid to reveal all the bits inside in the plastic tray. This nearly reduced me to tears the first time I saw it.