"That ... is" or "that ... be" [duplicate]

What matters most is that the merger be successful sounds better to me. The subjunctive mood shows that the merger hasn't happened yet and you are expressing a wish for it to be successful.

It's not really a matter of 'should'. In What matters most is that the merger is successful, is is indicative. In What matters most is that the merger be successful, be is subjunctive. This form of subjunctive seems to be found more in American English than in British English, where it creates a formal and rather prissy tone. An alternative that might be found in British English is What matters most is that the merger should be successful.