My preferred scale is dependent on period and rules.
I do Ancients (Home Brew Rules) and Napoleonics (SESWC DBA variant) in 6mm
Samurai big battle in 3mm (Home Brew) and Ninja skirmish with 28mm chibi
WW2 in 3mm (BKC), 6mm (Home Brew Rules), 10mm (BKC) and 15mm (Chain of Command)
Sci-fi RPG in 20mm
For me, 28mm are too ludicrously expensive, take up too much space (both on the table and in storage) and have too big a discrepancy between figure scale and ground scale for anything other than small skirmish games
3mm and 6mm allow for large numbers of figures per base to give a good “massed battle” feel.
10mm and 15mm give a decent balance between size, cost and detail.
20mm are big enough for players to identify with their figure.
My two ha’porth!
Growing old is mandatory, growing up is entirely optional!