According to this http://www.3character.com/priceguide.html .org, .mobi, .info then both .biz and .us
Depends where you are / what local your site content refers to ... .co.uk / .eu ...etc... also need consideration.
.co.uk is the 4th most popular worldwide and .de (Germany) is either the 3rd or 5th, the Germans go nuts for good domain names.
.mobi has been breaking records in terms of popularity, value, and mainstream acceptance (fox.mobi; bofa.mobi, hsm2.mobi, to show a diversity of industries). mtld.mobi is the registry where you can get the information about the gTLD that you need and both moniker.com (now merged with oversee.net) and GoDaddy both have reasonable prices ($10-12). .mobi has only been available for little over a year so there are still plenty of good, fresh registrations left, in addition to plenty of aftermarket sales.
.com -> .net -> .org -> .info -> .mobi -> .co.uk -> .cn Well I may be off a little, just go to Google.com and search: site:.ext (extension) this will show how many developed sites with that extension are listed in Google. The more, the more popular the extension IMO