Adobe Systems (ADBE) had the highest return in March 1999 by a mega-cap US stock, returning 40.7%.

ASSET	MONTH	% RETURN
Adobe Systems (ADBE)	March 1999	40.68%
Amazon (AMZN)	March 1999	35.58%
Taiwan Semiconductor Manufacturing (TSM)	March 1999	21.94%
Shell PLC ADR (SHEL)	March 1999	21.82%
Microsoft (MSFT)	March 1999	19.85%
ASML Holding NV ADR (ASML)	March 1999	17.07%
Chevron (CVX)	March 1999	15.63%
AstraZeneca PLC ADR (AZN)	March 1999	15.14%
Costco Wholesale (COST)	March 1999	13.57%
Toyota Motor Corporation ADR (TM)	March 1999	12.89%
Cisco Systems (CSCO)	March 1999	12.08%
Morgan Stanley (MS)	March 1999	11.35%
Johnson & Johnson (JNJ)	March 1999	10.32%
Bank of America (BAC)	March 1999	8.86%
Procter & Gamble (PG)	March 1999	8.67%
American Express (AXP)	March 1999	7.24%
UnitedHealth (UNH)	March 1999	6.91%
McDonald’s (MCD)	March 1999	6.42%
Exxon Mobil (XOM)	March 1999	6.41%
Home Depot (HD)	March 1999	6.24%
Walmart (WMT)	March 1999	5.92%
International Business Machines (IBM)	March 1999	4.57%
LIN TV (LIN)	March 1999	3.46%
Apple (AAPL)	March 1999	3.24%
Abbott Laboratories (ABT)	March 1999	2.74%

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