Weatherford International (WFRD) had the highest return between 2019 and 2020 by a mid-cap US stock, returning 1,471,575%.

ASSET	YEARS	% RETURN
Weatherford International (WFRD)	2019-2020	1,471,575%
Enphase Energy (ENPH)	2019-2020	3,689.85%
Axsome Therapeutics (AXSM)	2019-2020	2,799.29%
Plug Power (PLUG)	2019-2020	2,634.68%
Centrus Energy (LEU)	2019-2020	1,310.37%
TG Therapeutics (TGTX)	2019-2020	1,187.62%
Skeena Resources (SKE)	2019-2020	1,136.36%
Immunitybio (IBRX)	2019-2020	1,090.18%
Riot Blockchain (RIOT)	2019-2020	1,025.17%
SolarEdge Technologies (SEDG)	2019-2020	827.4%
Sitime (SITM)	2019-2020	761%
Quantumscape (QS)	2019-2020	757.36%
Magnite (MGNI)	2019-2020	741.37%
Acm Research (ACMR)	2019-2020	654.41%
Orla Mining (ORLA)	2019-2020	577.99%
Sunrun (RUN)	2019-2020	555.77%
Alphatec (ATEC)	2019-2020	542.48%
Arrowhead Pharmaceuticals (ARWR)	2019-2020	531%
Appian (APPN)	2019-2020	517.49%
DRDGOLD Limited ADR (DRD)	2019-2020	492.33%
Zai Lab (ZLAB)	2019-2020	488.95%
Sibanye Gold Ltd ADR (SBSW)	2019-2020	455.05%
Crispr Therapeutics (CRSP)	2019-2020	449.96%
Ocular Therapeutix (OCUL)	2019-2020	428.06%
Peloton Interactive (PTON)	2019-2020	423.17%

Mid-cap stocks are treated as having a market cap between $2B and $10B.