The oldest set that get an outing in the normal course of events for me is WRG 1925-1950 Armour & Infantry 1973 (Not the later edition which was ‘improved’ out of playability). It’s not the only set I use for WWII but I prefer it for company level games, and not just for the nostalgia value.
I used to play Gush’s/WRG Renaissance until a few years ago but tired of fiddling about with swapping figure bases about for casualties (the edges of the cardboard bases turning furry with handling after thirty years didn’t help) and I sold the figures and began the search for a different set of rules. I turned to Impetus for the Italian Wars end of the period and after several false starts have settled (I think) on Twilight of Divine Right for the end of it.
After that probably Volley and Bayonet, first edition is the oldest I use.
I have a confession which is going to get me drummed out of wargaming I know, but I have never played any Featherstone rules apart from a couple of skirmish games. Despite owning various books of his over the years they have never really appealed. I started with Charge! discovered Grant and then was directed to Featherstone’s which I found too simplistic and frankly a backward step I have never had a desire to take. Sorry!