Which is best professional digital camera in world?

Hassebald is the best but most pro photographers use
the Nikon D2X.. it's pricey too...

For professional photography, there is no substitute for a Hasselblad. Of course, the damned things cost the equivalent of the gross national product of a small country, which is why I use a Kodak. Click on the link to check them out.

It depends on what type of professional photography you are talking about. The Hassleblad H3D would be the one for studio type work, the new Canon 1DIII for sports, an small Olympus point and shoot like the C5050z would be great for photojournalism....I could go on.

Nikon D200

Canon Eos 30 D

check this link :

http://www.dpreview.com

my suggestion
go to yahoo shopping
digital cameras
digital camera GUIDE
be sure to check titles on the left side
the guide should answer your questions

to answer your question
you need more than $20,000

There is no best -- just best at a certain task.

Studio portraits/fashion? Mamiya RZ, Hasselblad, 8X10 if you want to be another Avedon

Studio commercial? Medium format with digital back

Sports? Any DSLR with 8fps

Photojournalism? Just about any DSLR with a wide angle lens

Travel? Still can't beat a Leica M6

Wedding? Mamiya 645, though many photogs are going for high-end digital (pity)

Hope this helps.