You can set a default email account in Settings/Mail, Contacts, Calendars... scroll down a bit and look for "Default account" under the Mail heading, however, I don't think that will do what you want.
Setting a default account simply lets iOS know which account to use when you click on a mail link on a web site or in an application.
I don't think there is a way to force the mail app to come up with a specific account each time you open it, but in my experience it remembers what you had open last time. So if you leave it showing all the accounts then it should return there next time you open it.