Amazon (AMZN) had the highest return in April 2001 by a mega-cap US stock, returning 52.8%.

ASSET	MONTH	% RETURN
Amazon (AMZN)	April 2001	52.76%
SAP SE ADR (SAP)	April 2001	48.16%
KLA-Tencor (KLAC)	April 2001	40.92%
ASML Holding NV ADR (ASML)	April 2001	30.07%
NVIDIA (NVDA)	April 2001	28.67%
Taiwan Semiconductor Manufacturing (TSM)	April 2001	27.92%
Lam Research (LRCX)	April 2001	26.63%
Applied Materials (AMAT)	April 2001	25.88%
Microsoft (MSFT)	April 2001	23.6%
Morgan Stanley (MS)	April 2001	18.69%
Intel (INTC)	April 2001	18.6%
International Business Machines (IBM)	April 2001	18.15%
Thermo Fisher Scientific (TMO)	April 2001	17.42%
GE Aerospace (GE)	April 2001	16.88%
Advanced Micro Devices (AMD)	April 2001	16.1%
Apple (AAPL)	April 2001	15.39%
Caterpillar (CAT)	April 2001	13.09%
American Express (AXP)	April 2001	12.07%
Micron Technology (MU)	April 2001	10.68%
Eli Lilly and (LLY)	April 2001	10.52%
UnitedHealth (UNH)	April 2001	10.5%
HSBC Holdings PLC ADR (HSBC)	April 2001	10.39%
Citigroup (C)	April 2001	10.33%
Johnson & Johnson (JNJ)	April 2001	10.26%
Chevron (CVX)	April 2001	9.98%

Mega-cap stocks are treated as having a market cap of $200B+.