Inventrust Properties (IVT) had the highest return between 2012 and 2015 by a mid-cap US stock, returning 751,855%.

ASSET	YEARS	% RETURN
Inventrust Properties (IVT)	2012-2015	751,855%
ACADIA Pharmaceuticals (ACAD)	2012-2015	3,140.91%
Cidara Therapeutics (CDTX)	2012-2015	2,045%
CleanSpark (CLSK)	2012-2015	1,053.86%
Patrick Industries (PATK)	2012-2015	953.25%
Travere Therapeutics (TVTX)	2012-2015	915.26%
Ligand Pharmaceuticals (LGND)	2012-2015	803.5%
Sarepta Therapeutics (SRPT)	2012-2015	751.66%
Ambarella (AMBA)	2012-2015	730.7%
American Airlines (AAL)	2012-2015	725.58%
Repligen (RGEN)	2012-2015	717.63%
Nexstar Broadcasting (NXST)	2012-2015	671.56%
Addus HomeCare (ADUS)	2012-2015	550.28%
Acadia Healthcare (ACHC)	2012-2015	536.05%
IES (IESC)	2012-2015	506.58%
TG Therapeutics (TGTX)	2012-2015	489.14%
Caesars Entertainment (CZR)	2012-2015	485.11%
EPAM Systems (EPAM)	2012-2015	471.78%
Radian (RDN)	2012-2015	468%
Western Alliance Bancorporation (WAL)	2012-2015	465.61%
Hanesbrands (HBI)	2012-2015	441.19%
Axos Financial (AX)	2012-2015	417.2%
HCI (HCI)	2012-2015	404.87%
TAL Education (TAL)	2012-2015	391.51%
Lithia Motors (LAD)	2012-2015	391.47%

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