What are LEGO's best selling products?
You can find some of The LEGO Group's financial information in their annual report. Note that the financials are in Danish kroner, so you'll need to do some currency conversion. For example, the net profit for 2012 was 5.6 billion DKK, which works out to about 1 billion USD.

I'm not aware of a reliable way to get official sales numbers for individual products or even product lines. The Mindstorms RIS 2.0 set was the best selling individual product as noted in this article from 2008:


  The Mindstorms RIS 2.0 is the #1 selling product in the company's history, and there are over 1 million RCX bricks that have been sold.


TLG does occasionally release some product sales information, as shown in this Brickset post.

If you are interested in the current bestselling sets, there is a page on LEGO.com with this information. Brickpicker also has a section for the current bestselling sets on eBay.