Likud will form the next government as part of a coalition that holds the majority of Knesset seats.
That's by far the most likely scenario; there aren't enough parties on the left and center to give Zionist Union a majority coalition, even with a 25 seat win. But a big win by ZU will force Netanyahu to include the hard right and the ultra-Orthodox parties in his coalition (which means he will lose Yesh Atid, who are center-right but totally unacceptable to the religious parties because of their insistence on drafting Yeshiva students into the Army). The result will be a coalition that Netanyahu will have trouble working with, giving him an unstable majority.