Of course, there are a huge number of artifacts and objects that are considered "priceless", but you've got to assume that some of these posessions are worth more than others. The question I'm asking is, what do you believe is the most valuable thing in the whole world (physical object, not a metaphor :p)

Here were some of the things that I thought of - still can't decide out of these which is worth more, and if I've forgotten something potentially even more valuable.

Crown Jewels: This crown alone (the Imperial State Crown) contains 2868 diamonds, along with the obvious cultural and historial relevence that would definitely increase its value

The Holy Chalice of Valencia: Proven to have been produced in a Palestinian or Egyptian workship between 400BC and 1st century AD, it's recognised by most historians (and by the Catholic church) to quite likely be the real Holy Grail. Obviously the Holy Grail, if real, would have massive value - would it be the most valuable object on Earth?

Again, nobody knows how much the Mona Lisa is actually worth, as it will never be put on sale. However, surely you can expect that people would pay very large amounts of money to be the owner of arguably the most famous painting in the world if it ever did go to market. Would it be worth more than the other objects on this list?

And of course, I'm open up for any alternative answers - I just thought up a few on the spot :) 

your health > material things

d21lewis' crown jewels

only physical objects allowed! :P

the most valuable thing i own is my hw/sw collection
much valuable than those

Apple.

No, really.

Although it could actually be the inner core of Earth - no, not the one you hear in school, the one below all that iron. A ball of some kilometers of diametre made of uranium, rhodium, gold, platinum and all that fancy stuff. It's unclaimed though, so that probably doesn't count.

Trinkets and junk will simply return to the dust from which they arose given enough time, regardless of the "value" which we place upon them. I know you said no metaphors but I believe that knowledge is the only thing that possesses any true value.

Humans.

Some people are willing to pay pretty much any price to save lives.

http://www.gametrailers.com/videos/nak99y/angry-video-game-nerd-angry-video-game-nerd--nintendo-world-championships