Par Pacific (PARR) had the highest return between 2012 and 2015 by a mid-cap US stock, returning 3,823.3%.

ASSET	YEARS	% RETURN
Par Pacific (PARR)	2012-2015	3,823.33%
ACADIA Pharmaceuticals (ACAD)	2012-2015	3,140.91%
Vipshop (VIPS)	2012-2015	2,444.97%
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	671.57%
Addus HomeCare (ADUS)	2012-2015	550.28%
IES (IESC)	2012-2015	506.58%
TG Therapeutics (TGTX)	2012-2015	489.14%
Caesars Entertainment (CZR)	2012-2015	485.11%
Radian (RDN)	2012-2015	467.98%
Western Alliance Bancorporation (WAL)	2012-2015	465.62%
Hanesbrands (HBI)	2012-2015	441.19%
Axos Financial (AX)	2012-2015	417.2%
HCI (HCI)	2012-2015	404.88%
TAL Education (TAL)	2012-2015	391.51%
Lithia Motors (LAD)	2012-2015	391.47%
Skyworks Solutions (SWKS)	2012-2015	362%

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