What type of battery is best to keep in a seldom used flashlight?
Don't use any sort of rechargeable cells.  They will be flat through self-discharge by the time you use them.
Non-rechargeable lithium cells aren't common in the size you want, but if you can source some, then they will last many years.
Otherwise, get brand new alkaline cells from a reputable manufacturer.  Check the &quot;install by&quot; date on the betteries or the packaging, to make sure they have several years left on them.  Even after their &quot;install by&quot; date, they will still work for a few more years, but will no longer last as long when you do use them.