Shell PLC ADR (SHEL) had the highest return in August 1986 by a mega-cap US stock, returning 40.4%.

ASSET	MONTH	% RETURN
Shell PLC ADR (SHEL)	August 1986	40.43%
Micron Technology (MU)	August 1986	26.58%
Toyota Motor Corporation ADR (TM)	August 1986	25.95%
Advanced Micro Devices (AMD)	August 1986	25%
Chevron (CVX)	August 1986	24.11%
Applied Materials (AMAT)	August 1986	19.43%
Apple (AAPL)	August 1986	18.86%
Home Depot (HD)	August 1986	18.71%
Raytheon Technologies (RTX)	August 1986	14.92%
Lam Research (LRCX)	August 1986	14.21%
Exxon Mobil (XOM)	August 1986	13.87%
American Express (AXP)	August 1986	12.13%
JPMorgan Chase (JPM)	August 1986	10.86%
Caterpillar (CAT)	August 1986	9.92%
Merck (MRK)	August 1986	8.31%
Johnson & Johnson (JNJ)	August 1986	8.27%
Bank of America (BAC)	August 1986	8.17%
GE Aerospace (GE)	August 1986	7.52%
Oracle (ORCL)	August 1986	6.53%
International Business Machines (IBM)	August 1986	5.5%
Eli Lilly and (LLY)	August 1986	4.46%
Wells Fargo (WFC)	August 1986	4.15%
Procter & Gamble (PG)	August 1986	3.39%
Berkshire Hathaway (BRK.A)	August 1986	1.4%
Microsoft (MSFT)	August 1986	0.83%

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