Vipshop (VIPS) had the highest return between 2012 and 2014 by a mid-cap US stock, returning 3,156.5%.

ASSET	YEARS	% RETURN
Vipshop (VIPS)	2012-2014	3,156.52%
ACADIA Pharmaceuticals (ACAD)	2012-2014	2,786.36%
QXO (QXO)	2012-2014	1,275.64%
Patrick Industries (PATK)	2012-2014	964.88%
Ionis Pharmaceuticals (IONS)	2012-2014	740%
TG Therapeutics (TGTX)	2012-2014	682.22%
Ambarella (AMBA)	2012-2014	655.89%
Radian (RDN)	2012-2014	608.8%
Nexstar Broadcasting (NXST)	2012-2014	571.25%
Acadia Healthcare (ACHC)	2012-2014	523.32%
Avis Budget (CAR)	2012-2014	497.57%
YY Inc Class A (YY)	2012-2014	493.71%
Repligen (RGEN)	2012-2014	472.25%
Hanesbrands (HBI)	2012-2014	406.42%
Globalstar, Inc Common Stock (GSAT)	2012-2014	391.07%
Axos Financial (AX)	2012-2014	377.95%
CleanSpark (CLSK)	2012-2014	374.36%
Skechers (SKX)	2012-2014	343.42%
Ligand Pharmaceuticals (LGND)	2012-2014	343.41%
Western Alliance Bancorporation (WAL)	2012-2014	338.48%
Jazz Pharmaceuticals (JAZZ)	2012-2014	330.64%
Mueller Water Products (MWA)	2012-2014	330.18%
Marriot Vacations Worldwide (VAC)	2012-2014	328.78%
Whirlpool (WHR)	2012-2014	327.3%
IES (IESC)	2012-2014	319.73%

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