I started in the early ’70s with 1/285 Micro Armour. Still have my first GHQ minis. I have kept consistent with 1/285, 1/300, or 6mm as my main scale. It is the only one for which I have collected buildings and most terrain types.
I have a lot of 25 mm fantasy (mass battle and RPG figures) and medieval mostly for mass battles. I don’t see anything about or referred to as 25 mm any more. I know about scale growth which I assume 28 mm really is. The only terrain is felt for roads and fields and the same lichen for forests. I have a handful of 25 mm sci fi from interesting figures from McEwan miniatures, Ral Partha and Citadel from back in the day and a couple Morrow Project figures as well.
Being a big Traveller and Striker fan I bought into the 15 mm sci fi craze mostly for some character figures and some others for NPCs and for small unit actions in Striker but 6mm was the main scale for anything over a squad per side. The only terrain is 1/2″ grid massive paper sheets (sold for use on an easel during meetings) for making starship and some ground based terrain much like snapshot or Azhanti High Lightening. Recently I have acquired more for use in skirmish battles, anything from modern to sci fi. I could do 6mm for skirmish but it would be awkward for handling.
I see rules “written for 28 mm” such as Chain of Command and Skirmish Sangin but feel changing from inches to centimeters and using 6mm would work better.
Almost forgot. I have a handful of 1/2400 CinC modern ships, some RAFM(?) unknown scale Traveler starships, unknown scale(s) of starships from many manufacturers from the ’70s and ’80s and a goodly collection of GHQ 1/1200 Micronauts of age of sail ships.