Oracle (ORCL) had the highest return between 1998 and 2000 by a mega-cap US stock, returning 679.3%.

ASSET	YEARS	% RETURN
Oracle (ORCL)	1998-2000	679.31%
Cisco Systems (CSCO)	1998-2000	305.3%
NVIDIA (NVDA)	1998-2000	212.22%
Amazon (AMZN)	1998-2000	211.24%
Morgan Stanley (MS)	1998-2000	179.86%
Micron Technology (MU)	1998-2000	173.08%
Applied Materials (AMAT)	1998-2000	152.48%
UnitedHealth (UNH)	1998-2000	147.12%
Home Depot (HD)	1998-2000	134.39%
Raytheon Technologies (RTX)	1998-2000	123.83%
Apple (AAPL)	1998-2000	118.22%
Taiwan Semiconductor Manufacturing (TSM)	1998-2000	117.26%
GE Aerospace (GE)	1998-2000	103.69%
ASML Holding NV ADR (ASML)	1998-2000	99.08%
American Express (AXP)	1998-2000	89.08%
Merck (MRK)	1998-2000	86.64%
Costco Wholesale (COST)	1998-2000	78.99%
Intel (INTC)	1998-2000	70.87%
Johnson & Johnson (JNJ)	1998-2000	67.81%
International Business Machines (IBM)	1998-2000	65.38%
Wells Fargo (WFC)	1998-2000	54.55%
Berkshire Hathaway (BRK.A)	1998-2000	54.35%
Berkshire Hathaway (BRK.B)	1998-2000	52.96%
Advanced Micro Devices (AMD)	1998-2000	51.37%
Lam Research (LRCX)	1998-2000	47.46%

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