Viatris (VTRS) had the highest return in August 1989 by a large-cap US stock, returning 50.6%.

ASSET	MONTH	% RETURN
Viatris (VTRS)	August 1989	50.63%
Kinross Gold (KGC)	August 1989	38.89%
Telefonaktiebolaget LM Ericsson B ADR (ERIC)	August 1989	29.47%
Cooper Companies, Inc Common Stock (COO)	August 1989	26.3%
Expeditors International of Washington (EXPD)	August 1989	24.15%
ConocoPhillips (COP)	August 1989	23.89%
Oracle (ORCL)	August 1989	22.97%
Bio-Techne (TECH)	August 1989	20.88%
Lowe's Companies (LOW)	August 1989	18.41%
Church & Dwight (CHD)	August 1989	17.04%
Genesis Healthcare (GEN)	August 1989	16%
Dupont De Nemours (DD)	August 1989	15.47%
Ecolab (ECL)	August 1989	15.11%
US Bancorp (USB)	August 1989	15.09%
UnitedHealth (UNH)	August 1989	14.81%
Laboratory Corporation of America (LH)	August 1989	14.49%
KLA-Tencor (KLAC)	August 1989	14.28%
Nike (NKE)	August 1989	13.64%
Home Depot (HD)	August 1989	13.55%
Halliburton (HAL)	August 1989	12.85%
Progressive (PGR)	August 1989	12.42%
Aflac (AFL)	August 1989	12.39%
International Paper (IP)	August 1989	12.32%
W R Berkley (WRB)	August 1989	12.23%
Apple (AAPL)	August 1989	12.21%

Large-cap stocks are treated as having a market cap of $10B+.