None. I was asked recently what were good digital advertising conferences to go to. But I actually think most of them are rubbish.
You have the same people saying the same stuff that everyone has been saying for 3 or 4 years. Or even worse new people saying the same stuff as if they invented  it. Often professional speakers doing the circuit or people in agencies trying to push their own personal agenda.
Generally it is a bunch of hacked presentations showing really interesting/engaging/disruptive/social (delete as appropriate) work and saying that what we need to do and brand/advertising/social/digital/direct (deleted as appropriate) are the best agency to deliver it.
But that is lazy. It is sodding easy to bash advertising and show examples of cool  stuff we should be doing. Very few people actually offer constructive  advice on how we need to change our process as well as our product.  Global marketing business re-engineering. That's what interests me and I have had a bash at it here.
So don't go to a conference spend 3 hours on slideshare.com, follow the right people on twitter and think & talk alot. Alot, a lot.
But if you do fancy a conference, here is a list one ones my good Ogilvy friend Nicole Yershon recommends. Attend or follow the buzz online.
As you can see most of them are not about advertising but look at music, film, arts, technology and business.
UK Specific
 thinking digital - 24-26 May in Sheffield
 TED salon events - London 3 times a year
 google zeitgeist - May in UK
 click - Nov in London
 power to the pixel - Nov in London
 media pro - Nov in London
 sounds digital - Apr in London
 the childrens media conference - June in Sheffield
 the sheffield doc fest - June in Sheffield
 PSFK global events -  London & Global
 iStrategy - London & Global
 likeminds - Nov in Exeter & Lon
 b.tween - UK
 Global Economic & Political
(that all have digital/tech on their agenda)
 World Economic Forum - Jan in Davos
 DLD - Jan in Munich
 TED - All the time, everywhere
 Clinton Global Initiative - Sept in NY
 Business
(that all have digital/tech  on their agenda)
 allen & company sun valley conf - July in Idaho
 all things D - June in San Diego)
 monaco media forum - Nov in, err, Monaco
 berkshire hathaway annual shareholders mtg - May in Omaha
 summit series - Apr in USA
 Global Digital, Tech & Innovation
 techcrunch disrupt - Spet in San Fran
 le web - Dec in Paris
 the lobby - Sept in Hawaii
 fortune brainstorm tech - June in Aspen
 sxsw - Mar in Austin
 CES - Jan in Vegas
 picnic - Sept in Amsterdam
 IBC - European CES - Sept in Amsterdam
 WEB 2.0 - Nov in San Francisco
 miptv - Apr & Oct in Cannes
So I have probably missed afew out and I am sometimes happy to be proved wrong. If someone fancies inviting me to attend or speak, after all I have my personal agenda to pursue and a couple of borrowed decks I can hack together...