Firstly Barak is not standing so he is out the picture secondly if there is no clear winner the president choses the candidate most likely to be capable of forming a coalition as we saw last time with Kadima defeating Likud by 1 seat but Peres chose to make Bibi PM because it was thought he had the best chance of forming a coalition. Bibi could easily get the traditional right wing and religious parties to join him in a coalition and I can't see Lapid liking the idea of sharing a coalition with Shas much. There is also the issue of is any of them capable of being PM? Shelly been in politics 5 years, never sat in a serving government & has only been party leader 1 year so is far from qualified, Lapid he has never even been politics before and so not the kind of man you want to go straight in as PM, Eli Yishai of Shas has plenty of experience but I can't see many people wantinga haredi as a PM. Livni has some experience as both a minister and a party leader although it should be remembered she is the one most responsible for taking Kadima from hero-zero and her policies seem to be a little bit wishy washy. Last but not least of course if it were even possible to get Labor, Lapod, Livni, Shas and Meretz together in a single coalition they would still get less seats that a coalition of just Likud & Shas and you add into that the nationalists Bibi can easily form a coalition and based on his experience as a PM, variety of ministries, number of years he has been in politics and his life experiences before politics even if you hate him and his policies he is easily the most qualified man for the job.
from the article: What goes up must come down? Haaretz brings you the latest polls