Series, Parallel or Series/ParallelBbattery Arrangement Best or Safest?

This is in reference to Lithium Ion rechargeable batteries. I note that older LED flashlights typically used their batteries in series, stacked in a long and skinny battery tube. So did most incandescent flashlights. More recently the short stubby flashlight has become popular and many of the 18650 battery versions have the batteries set up so that they are in parallel. Some of the most powerful lights use a series/parallel arrangement such as the battery pack Fenix makes for the RC40 which I believe is a 2S3P arrangement of 6 cells as is the Olight pack for their large SR series lights.

From an engineering or safety standpoint is there any particular advantage to one arrangement over another?

Series increases voltage, allowing less current for a given watt draw. Parallel increases current, allowing less voltage for a given watt draw. Series-parallel achieves both increased voltage and current capacity.

What's better? Depends on application. Until recently, LED flashlights could not exceed current delivery capabilities of most lithium cells, thus a series stack was adequate. We're seeing series-parallel stacks because the designs now require it to allow most common chemistries to perform.