At the very top of the stupidity chain (the czar of drooling incompetence) a baboso is someone who doesn't have enough personality to be a full-blown pendejo.
The beauty of this self-explanatory insult resides on its unparalleled bashing power while remaining an all-audiences term unless dressed up with spicier words, usually:
*User discretion advised
It can also be mellowed for a more collegial use:
No seas babas!