After you boot into the lightest version, run Windows Update blindly. Why? The "Telemetry" updates (KB3022345 and KB3068708) added in 2015 add massive background services to Windows 8.1 to make it compatible with Windows 10's data collection. These updates increase RAM usage by 150MB.
Windows RT 8.1 is the lightest version only if you want a very heavy paperweight. windows 8.1 lightest version