Vipshop (VIPS) had the highest return between 2012 and 2014 by a mid-cap US stock, returning 3,156.6%.

ASSET	YEARS	% RETURN
Vipshop (VIPS)	2012-2014	3,156.64%
ACADIA Pharmaceuticals (ACAD)	2012-2014	2,786.36%
Aurinia Pharmaceuticals (AUPH)	2012-2014	2,326.67%
Patrick Industries (PATK)	2012-2014	964.88%
TG Therapeutics (TGTX)	2012-2014	682.22%
Ambarella (AMBA)	2012-2014	655.89%
Radian (RDN)	2012-2014	608.8%
Addus HomeCare (ADUS)	2012-2014	577.93%
Nexstar Broadcasting (NXST)	2012-2014	571.26%
Travere Therapeutics (TVTX)	2012-2014	544.21%
Saia (SAIA)	2012-2014	542.23%
HCI (HCI)	2012-2014	509.36%
Avis Budget (CAR)	2012-2014	497.57%
JOYY (JOYY)	2012-2014	493.72%
Repligen (RGEN)	2012-2014	472.25%
Pilgrims Pride (PPC)	2012-2014	453.88%
Globalstar, Inc Common Stock (GSAT)	2012-2014	391.07%
Axos Financial (AX)	2012-2014	377.95%
CleanSpark (CLSK)	2012-2014	374.36%
Ligand Pharmaceuticals (LGND)	2012-2014	343.41%
Western Alliance Bancorporation (WAL)	2012-2014	338.49%
Skyworks Solutions (SWKS)	2012-2014	333.22%
Jazz Pharmaceuticals (JAZZ)	2012-2014	330.64%
Mueller Water Products (MWA)	2012-2014	330.15%
Marriot Vacations Worldwide (VAC)	2012-2014	328.78%

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