Amazon (AMZN) had the best month in 1997 by a large-cap US stock in September 1997, returning 85.1%.

ASSET	MONTH	% RETURN
Amazon (AMZN)	September 1997	85.07%
Sterling Infrastructure (STRL)	October 1997	79.99%
Regeneron Pharmaceuticals (REGN)	May 1997	70%
Advanced Energy Industries (AEIS)	July 1997	62.6%
Onto Innovation (ONTO)	August 1997	60.9%
Insmed (INSM)	January 1997	55.53%
Amazon (AMZN)	July 1997	55.38%
Incyte (INCY)	May 1997	54.12%
Stifel Financial (SF)	June 1997	53.23%
Skyworks Solutions (SWKS)	July 1997	50.38%
Insmed (INSM)	May 1997	50%
Ciena (CIEN)	May 1997	49.45%
ASML Holding NV New York Registry Shares (ASML)	January 1997	47.73%
Best Buy (BBY)	September 1997	46.3%
Semtech (SMTC)	January 1997	45.26%
Lam Research (LRCX)	January 1997	42.91%
Advanced Energy Industries (AEIS)	January 1997	42.85%
Tower Semiconductor (TSEM)	July 1997	42.71%
Lam Research (LRCX)	July 1997	41.95%
Jabil Circuit (JBL)	June 1997	41.56%
NetApp (NTAP)	December 1997	41.29%
MasTec (MTZ)	May 1997	40.09%
Advanced Energy Industries (AEIS)	May 1997	40%
Amphenol (APH)	June 1997	39.48%
Qualcomm (QCOM)	January 1997	39.13%

Large-cap stocks are treated as having a market cap of $10B+.