We never actually see the Most Fabulous Object in the World, mostly because it doesn't actually exist, but the dwarves sure are keen on getting it. Evil uses the idea to lure them into the Fortress of Ultimate Darkness, and once there, he figures—correctly—that he can nab the map from them without much of a fuss.
What's telling isn't what the object actually is but instead how Evil chooses to present it. He pulls out the showroom from that inane game show Kevin's parents watch—complete with the fiendishly grinning host, and with Kevin's parents done up in spangly showbiz duds—to lure in the dwarves. It's crass, it's shallow, it's materialistic. Basically, it's everything the Most Fabulous Object in the World should not be.
Seriously. We're pretty sure the ultimate treasure in the universe wouldn't resemble a set of kitchen appliances.
But that's the way Evil thinks—and unfortunately, it's the way the dwarves think, too. Only Kevin sees through the ruse, making this simple non-object a very subtle way of commenting on the things we value, and why. The dwarves figure that out too late. Luckily, they've got a smart kid watching their backs.