I am assuming that you are talking about from an advertisers view point. Based on that Bing ads has been the best one that has performed the best per dollar spent on advertising for me. Since clicks for advertisers using Bing cost less than Google I am able to get more clicks for the same amount of money.
Yes Bing gets less users than Google does but since you are paying for clicks and not views it works better and your money will net you more clicks for each dollar that you spend.
Cheers, Dean.