I prefer to use the excellent ‘God of Battles’ system by Jake Thornton for my bigger mass-battle affairs which I usually play with my 15mm armies. We even got a bit of a community going here in Denmark and there have been talks about a grand get together soon. My only caveat – and niggle – in regards to GOB is that it is a bit too limited in its approach to Fantasy; it’s firmly set to the ranges that Foundry produced/produces and thus it ends up being a bit narrow in its scope.
For my more skirmishy affairs I tend to use my own homebrew skirmish set that I’ve been working on/off over the years. It’s finally getting done after many a revision.