What is the most overlooked item during preflight inspection?
This FAA article is about using electronic checklists for airline flights (it concludes that they're better than paper ones) and includes this quote:


  They found the crews averaged 3.2 checklist errors per flight (one of
  the observed flights had 14.) The most-common checklist error involved
  the omission or deferral of an item, which was later forgotten.


NASA commissioned a general study into checklists in 1990, but it doesn't identify specific items that are commonly overlooked. It does say that as the number of items on a checklist increases, the chances of overlooking any individual one does too:


  Swain and Guttman (1983), in their study of nuclear power plant
  operations “recognized the fact that as the list of items grows, there
  may be a higher probability of overlooking any given item” (chap. 15,
  p. 13)


The final section of the paper has recommendations for checklist design, which could be considered the "best practices" you mentioned, but of course it's a very general list, e.g.


  The most critical items on the task-checklist should be listed as
  close as possible to the beginning of the task-checklist, in order to
  increase the likelihood of completing the item before interruptions may occur


AOPA's Flight Training magazine has an article on "frequently forgotten" checks that is more specific and most of the checks seem to apply to most (light) aircraft, but it doesn't provide any authoritative sources. They also have a "how to preflight" article that gives more information and includes this interesting observation:


  As an experiment, I held a preflight contest with a "bugged" trainer.
  I reversed the rigging on the ailerons so their movement was contrary
  to control inputs. Every one of the contestants checked control
  movement and freedom, but no one caught the problem.


Putting all that together suggests - only to me, and I'm not an expert - that it's less important to think about specific items that you might forget, and more important to ensure that you complete the checklist fully and really verify the results of each check. If you consider some checklist items to be 'more important' than others then it could also lead to mistakes because you overlook the 'less important' ones.