I love the idea of this thread. There're a few vehicles I'd like to see, some real, some not.

Real vehicles: I would have liked to have seen the Eagle lunar landing module as a TF and I could also see a version of Prowl as a 50's cop car.

Non-real vehicles: probably more like franchise crossover vehicles though. Ecto 1 has already been mentioned and the fanboy in me would also like to see the spaceship and drop ship from the movie Aliens, but the vehicle I'd most like to see as a TF is Space battleship Yamato (known as the Argo in Starblazers for Western audiences) transforming into a samurai robot. Pipe dream I know, but I'd love to see it!