You'll never get an answer to this question that comes with a 100% guarantee of accuracy. Billionaires don't publish their bank accounts and valuations of their properties and business interests can be rather subjective (as well as subject to fluctuating share prices and exchange rates).
The Sunday Times publishes an annual 'Rich List'. It's not really a single list but a whole series of tables. Their 2006 worldwide table puts Robert Walton (WalMart) in 1st place, with Bill Gates 2nd and Warren Buffet 3rd. You can download the list of the richest 100 people in the world (as a pdf), together with the corresponding list for Europe, here:
http://business.timesonline.co.uk/section/0,,2 9049,00.html
(There's lots of other interesting information on that page).
If you can't afford the postage to send your begging letters overseas, you can find the information for Britain & Ireland, here:
http://business.timesonline.co.uk/section/0,,2 0590,00.html
Chris