Alcoa (AA) had the highest return between 1958 and 1965 by a mid-cap US stock, returning 17.2%.

ASSET	YEARS	% RETURN
Alcoa (AA)	1958-1965	17.21%
Goodyear Tire & Rubber (GT)	1958-1965	6.74%

Mid-cap stocks are treated as having a market cap between $2B and $10B.