Steel, as in the alloy? Or, do you mean tubing profile/detail/diameter?
And, what are the limits on vintage? Today's alloys (853, 953, and the latests from Columbus have better strength to weight (for the alloy, this IS the key metric) ratios than the 531 alloy or Columbus Cyclex (what they used for SL, SLX, SP, SPX). But of the vintage stuff that was around 25 years ago, best strength to weight has to go to 753 and possibly Excell.
Tensile strengths:
Excell: 200k lb/sq.in.
Reynolds 753: 179k lb/sq.in
Cyclex: 121k lb/sq.in
Ishiwata: 113k lb/sq.in
Tange (1,2, and 3): 129k lb/sq.in
From "Bicycle Metalurgy," Douglas Hayduk, 1987.
Or is it how tubes were designed using a particular metal?