G'day MF and Max
That's a great question there MF.

If we're talking 1970s rear-catcher lawnmowers then I feel
Victa, Rover, Scott Bonnar and Supa-Swift were at the top
of their game. They were all quality lawnmowers built to last.

The things that stood out for me for Supa-Swifts were: -
[1] fluted and stepped blades;
[2] quality die cast handle height adjusters;
[3] a super robust chassis and cut height adjuster.

I feel any call on the 'best' would be subjective.

Max and Yourself would be better judges here.

Cheers
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Jack