My Hornby Advanced Passenger Train, if I'd been allowed to play with it. My father bought it for xmas but the APT project had been cancelled (but later sold on and used successfully) he thought it'd be rare and worth something so I wasn't allowed to play with it except fewer than a handful of times.
I still have one boxed, hardly used, Hornby APT and am open to sensible offers.
It's also a good job I double checked my phone's autocorrect of this post or else I'd have a Horny APT, and that's just scary!
The toy that I was allotted to play with, and I certainly got my money's worth out of was my Millennium Falcon (and other SW stuff).