20mm is my favorite skirmish scale. Lots of rank n file choices in plastic for the periods I am interested in. I can supplement these with metal. I am working on AWI right now in metal.
For big battles, anything from 15mm down through 6mm though the latter is kind of hard for me to see, at least when it comes to WW2 type games.
I do envy people that dabble in the large selection of 28mm figures now and it does seem they spoiled for choice. But 28mm is pretty expensive. Maybe I would have done more back in the day when metal prices were quite low.
"Nearly all men can stand adversity, but if you want to test a man's character, give him power."