The source of every default AOSP wallpaper

Android Milestone 3: Wikipedia, Aurora Borealis / Wikipedia, Moraine Lake

Note that Milestone 3 uses cropped versions of those images. They both look decent.

Android Milestone 3 (sunset)-Milestone 5-0.9: Flickr

This one is beautiful.

Android 1.x-4.0.3: Android source code

Honestly, this one looks very washed out and overdone IMO.

Android 4.0.4: Android source code

Looks very Googley.

Android 4.1-6.0: Android source code

It looks quite nice, actually.

Android 7.0-12.0: Android source code

I've seen this one countless times, but it looks quite nice too.

Android 12.1-14: Android source code

It looks decent.

Current: Android source code

As of writing this, same as 14, but it might change.