I'm glad you didn't limit it to just one item as that would simply be too difficult for any of us

The ones on my list that are really near-impossible to get hold of (primarily because of the price) include:
- the cancelled 5th series of the 1990s Iron Man figures by ToyBiz. This series includes Dark Aegis, Radiation Armor IM, Lava Armor IM, Living Laser and Magnetic Armor IM. A few pre-release final products did make their way to some people and some even got listed on eBay over the past few years.
- Mego Superman, Jor-El, Luthor and General Zod - these are the 12-inch figures which were released around the same time the first Christopher Reeve movie was first shown.
- Mego Batman w/ removable cowl - simply because it's Batman.
- the entire Super Powers line - given that I'm a person obsessed with completing the currently running JL/JLU toys, wanting to have SP seems like a natural tendency.
I do have a lot more to list but they are mostly antiques so I'll just stick with those that are still in the realm of possibilities.
As the great philosopher Wayne Campbell once said, "It will be mine. Oh yes. It will be mine...."