Fortress Maximus, Complete, MISB.
I have always wanted this since childhood and I saw it in the pack-in catalog. I hadn't seen him on the show until I bought the Rhino release of G1, season 3 part 2 and season 4.
He shall always be the giant fish that got away.
And since I saw all the extra goodies that came with the Takara release, now THAT one is my holy grail.