If I had to name just one set of rules that made me the wargamer I am today it most probably would have to be these, not that I expect many people have heard of them or played them...
Moving swiftly on from HO/OO I soon discovered the 'Joy of Six'(mil) with fellow six-formers Simon and Graham. As posted a few years ago my first proper wargames army was 6mm WW2 US armour mainly Heroics & Ros with a smattering of Skytrex and the odd Leicester Micro-model. I don't recall where the rules came from but they were the ones we used for the best part of two years of weekend gaming in Graham's garage.
I fell in love with the percentage based to hit mechanism and used variations of it in my own rules such as Iron Cow and even Flintloque!
I haven't played 6mm WW2 for decades now but these rules and the games we played with them were the foundation stones on which my hobby is based.