In Preview, how to set 'Rectangular Selection' as the default (NOT 'Text Selection'
This is not a "permanent" change, but it speeds up the inconvenience of needing to use the "Tools" menu, by adding a keyboard shortcut:

Here is the first option for something you can do:

CREATING A KEYBOARD SHORTCUT


Open System Preferences
Click "Keyboard"
Open the tab titled "Shortcuts"
Scroll down to the last item on the left-hand frame's list: "App Shortcuts"
Click on the "plus sign" [+]; a smaller window will pop-up, asking for three lines of information:


— Select "Preview" for the application

– For "Menu Title", you must type EXACTLY what appears in the "Tools" menu of Preview: for OS El Capitan (Mac OS 10.11), this is currently "Rectangular Selection" (don't include quotation marks)

— For "Keyboard Shortcut", use whatever combination you want, so long as it does not conflict with any other pre-existing or currently used shortcuts. As an arbitrary example, I use "Command+Shift+R". Invent whatever alternative you might like.


Just a SIDE-NOTE: if you repeatedly press your keyboard shortcut [which you define in step 5], I have found that Preview will switch back to the "Text Selection" tool [when it is available], which might be occasionally [rarely!] convenient... since, (like you), I too wish that Rectangular Selection were the default... (especially for cropping, etc.)


Here is a second option, involving a "built-in" keyboard shortcut, combined along with one mouse-click [instead of the two required if using the "Tools" menu]:


Press "Command+Shift+A" to make the sub-toolbar, "Markup Toolbar" appear or disappear; [note: this toolbar has the rectangular selection option on the left].
With sub-toolbar available [see step 1], click the 2nd item in from the left, to enable rectangular selection.


Hope that helps!...

Note that, for some reason, I have not always had success with the 1st Option [programming an app-specific shortcut using System Preferences] in certain versions of Mac OS or with particular apps -- even though the menu item might be spelled precisely and correctly as it appears. It was thus pleasantly surprising to find that this custom keyboard-shortcut option indeed works under El Capitan [OS 10.11], as of the time of writing this here.

Hope this helps others!