The best ever OS .... has to be GEORGE3, it was at least 2 generations ahead of its contemporaries. It had Virtual Memory and Virtual Filestore and sophisticated workload scheduling, the OS ran the operators and some very large businesses.... and that was back in the 1960's when things were measured in K's not M's and G's were never even dreamt of and tape was king because no one could afford those new-fangled little disc things (which weighed in at about 32lbs a piece). But it wasn't American and went the way of the British Empire. On Virtual Memory.... when IBM 'invented' it for OS360 they ran an ad .... "Tomorrow came Today to which ICL responded with a similar ad "No, Tomorrow came Yesterday".