...is generally accepted to be the Ferranti Titan.
The ICL 1900, which was launched in 1964 alongside the IBM System/360, had virtual addressing from launch. From the start 1900s used virtual addressing for memory and peripherals, though all programs were started manually by an operator. By 1970 my site was running a full-time workload of interactive and batch jobs under George 3.
Meanwhile, the System/360 was supposed to support virtualisation but this never worked. As a result it was stuck with DOS* and MFT or MVT/HASP until 1970 and the release of System/370 with VM/370 before it had a usable virtual memory system.
* NO, this was NOT PC-DOS (later rebadged as MS-DOS). That didn't appear until 1981.