Freedom (FRHC) had the highest return between 2014 and 2020 by a mid-cap US stock, returning 732,900%.

ASSET	YEARS	% RETURN
Freedom (FRHC)	2014-2020	732,900%
Inventrust Properties (IVT)	2014-2020	465,880%
Weatherford International (WFRD)	2014-2020	55,963.81%
Enphase Energy (ENPH)	2014-2020	2,658.96%
Ringcentral (RNG)	2014-2020	1,974.27%
TAL Education (TAL)	2014-2020	1,858.16%
Green Brick Partners (GRBK)	2014-2020	1,846.92%
Globant (GLOB)	2014-2020	1,775.95%
Plug Power (PLUG)	2014-2020	1,546.12%
SolarEdge Technologies (SEDG)	2014-2020	1,495.6%
Appfolio (APPF)	2014-2020	1,381.81%
Caesars Entertainment (CZR)	2014-2020	1,356.27%
Applied Digital (APLD)	2014-2020	1,308%
Repligen (RGEN)	2014-2020	1,294.69%
Appian (APPN)	2014-2020	1,250.75%
TG Therapeutics (TGTX)	2014-2020	1,213.64%
Quantum Computing (QUBT)	2014-2020	1,075.83%
RadNet (RDNT)	2014-2020	1,064.88%
Goosehead Insurance (GSHD)	2014-2020	968.68%
Casella Waste Systems (CWST)	2014-2020	968.1%
Novanta (NOVT)	2014-2020	955.54%
EPAM Systems (EPAM)	2014-2020	930.33%
Crispr Therapeutics (CRSP)	2014-2020	920.73%
Acm Research (ACMR)	2014-2020	894.47%
Amicus Therapeutics (FOLD)	2014-2020	882.55%

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