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	679.33%
Cisco Systems (CSCO)	1998-2000	305.29%
NVIDIA (NVDA)	1998-2000	212.22%
Amazon (AMZN)	1998-2000	211.24%
Morgan Stanley (MS)	1998-2000	179.86%
Walmart (WMT)	1998-2000	173.18%
Micron Technology (MU)	1998-2000	173.08%
UnitedHealth (UNH)	1998-2000	147.12%
Home Depot (HD)	1998-2000	134.39%
Raytheon Technologies (RTX)	1998-2000	123.3%
Apple (AAPL)	1998-2000	118.28%
Taiwan Semiconductor Manufacturing (TSM)	1998-2000	117.27%
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%
Johnson & Johnson (JNJ)	1998-2000	67.81%
International Business Machines (IBM)	1998-2000	65.38%
AstraZeneca PLC ADR (AZN)	1998-2000	57.23%
Abbott Laboratories (ABT)	1998-2000	54.86%
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%

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