Hmm, good question. It is tough for me to pick just one. Here are three off the top of my head that I would consider are all in the running to be my current holy grail would be the Pallisades' Micronaut's re-issued
Force Commander (white or clear red versions) with his Stallion, Oberon. They were only given to people who had joined the Pallisades' collectors club. They are hard to come across and when you do find them they all are priced from $100-150.
I guess I'll also add the Pallisades' Micronauts re-issued
Gold Baron Karza figure with his stallion, Andromeda. This figure was a very limited edition given only to microman/micronaut collectors who had volunteered to give their vintage Baron Karza figures to Pallisades in order to help them in creating the new reproduction Baron Zarza figure. These gold figures were a "thank you" sent only to the collectors that contributed old figures to Pallisades.
Lastly, I'd include the
Microman Rescue Secret Base. It is pretty tough to find and when you come across it the seller is always asking $300 or more for it. I just have a tough time shelling out that much for one toy.