I think if I could go anywhere in the world it would be to one of the many natural wonders of the world that may not be here in 50 years (not creating a debate here) or that are just amazing.

That being said, I think you need to get an idea on what they want to do in order to pick out a great place. Bora Bora, Maldives, Seychelles, Kauai - all are beaches and islands. I would love them but as mentioned a beach is a beach.

Do they like to see the sights, explore natural wonders, just relax? I think the experience of a dream vacation is often more important than the actual place. Of course, most of these wonderful places make the experience. But I think you know what I mean.

Now that I have kids, most of my dream vacations are different from when I was in my 20s and all involve the kids learning while we do. I keep saying "when the kids are older they would enjoy that more."

so they key really is what would they want to do.

Top of my list would be:
China (big country, I know)
India
African Safari
Morocco (cheap, wonderful vacation to be had for all!)
Galapagos
Italy and Slovakia to visit the homelands of my ancestors.
Bora Bora/Maldives/
Cuba
Japan
Egypt
.... list goes on and on