By Default, Outlook sets the order of Address Books in which the Global Address List (GAL) is first. Below are steps one can follow to change which Address is selected by default when hitting TO: to choose a recipient:
For Outlook 2013 & 2016
On the Home tab, in the Find group, click Address Book.
In the Address Book dialog box that opens, click Tools > Options.
Within The Tools menu under "When opening the address book, show this address list first", choose the name of the address book you want to access first by default.
Here you can also change the order the address books are listed within the drop down.
Click OK, and close the dialog box.