My vote goes for AdSense (for the foreseeable future). There is already a very large advertising client base with AdWords, and while some will also use Overture and/or MSN, it may take many months or even years for the majority to move over.
I don't see chitika do that well long term and Yahoo! will most likely be a little bit under Google all the time. The question is whether msn will start living up to its expectations when it comes to the search engine and affiliate program sector. I see a possibility of msn being on top a few years from now, but since I'm skeptical, I vote for Adsense.
AdSense will continue to dominate merely because of the Google name, marktet saturation, public trust factor, name recognition, pop culture saturation (how many times a week do your hear Google on a TV show), and many other factors. That's not to say that they can't be overcome, but it will take time, money, and possibly even scandal before people don't naturally migrate to Google.
If google bring technical advancement to their system to protect the webmasters from fraud clicks, I think their future is bright. Otherwise chitika might win the race.
I am going to stick my neck out and disagree with that seems to be the consensus. I think msn will win in the end. Google have made a lot of mistakes, which is to expected from such an experimental idea, which was largely their own. However, I think while yahoo have just copied Google I think msn will learn from Google's mistakes and provide a service that is better than Google's. It will indeed take time for people to convert, even if the service is better. But eventually people will leave google and go to msn or some new player imho. Whatever happens I am glad google is finally meeting some stiff competition. I have nothing against google, they have come up with some genius stuff, but I just think msn will come out on top, through watching, waiting, looking and learning.
Adsense, for the present. Of course, the problem with being the biggest is the eventual fall into thinking you're the best just because you have no real competition. Then a new technology will take ol' Google down, but that's not forseeable right away.
I'll go with Adsense. They already have a tried and tested formula. With the innovative team at Google constantly adding new things I'm sure they will stand the test of time. YPN might lure alot ot publishers their way with the higher payouts, but once it's out of Beta, I'm sure that their bubble will burst and publishers will start moaning about low payout.
Adsense, for atleast a few years to come. Maybe MSN can take over, but they need to improve their SE a lot.