I believe that we all have an idea of how big we want our game collection to grow, what systems we want, et cetera. But what is that one game that you want for your collection? What game is it that would be as an edifice, standing before your entire collection, that defines what you have been working up towards after all those years of gaming?
The game I would eventually like to own would be a Space Harrier arcade cabinet (preferably the sit-down version). Here's a pic I found:

I freakin' love Space Harrier. I was first introduced to it when walking into the arcade in Shenmue. I played darts and then I saw this arcade game... Space Harrier? I popped in some virtual yen and fell in love instantly. I now have a couple different home console ports of the game but I really need the arcade experience in my home. (The only downside to this is that I'd probably get addicted and end up buying more arcade machines. Hang-on, anyone?)
I can't afford and/or store it right now but it's one of my after-college goals to get my very own Space Harrier arcade cab. Now that I think about it, I guess my goal of owning my own home should come a little before that.
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So what's your game and why? Or if it is a game related object (like a hard to find promotional poster, etc.) that's fine as well. (Or are you going to be pushing the boundaries of reality in search of that solid gold cartridge of Ballz 3D?)