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	2,445.05%
Patrick Industries (PATK)	2012-2015	1,479.9%
Travere Therapeutics (TVTX)	2012-2015	915.26%
Himax Technologies (HIMX)	2012-2015	853.58%
Celsius (CELH)	2012-2015	823.86%
Ligand Pharmaceuticals (LGND)	2012-2015	803.5%
Ionis Pharmaceuticals (IONS)	2012-2015	742.59%
Ambarella (AMBA)	2012-2015	730.7%
American Airlines (AAL)	2012-2015	725.58%
Nexstar Broadcasting (NXST)	2012-2015	671.56%
Addus HomeCare (ADUS)	2012-2015	550.28%
Acadia Healthcare (ACHC)	2012-2015	536.05%
JOYY (JOYY)	2012-2015	494.95%
Celldex Therapeutics (CLDX)	2012-2015	491.7%
Caesars Entertainment (CZR)	2012-2015	485.11%
EPAM Systems (EPAM)	2012-2015	471.78%
Radian (RDN)	2012-2015	467.98%
Western Alliance Bancorporation (WAL)	2012-2015	465.62%
NeoGenomics (NEO)	2012-2015	454.23%
Builders FirstSource (BLDR)	2012-2015	427.62%
Axos Financial (AX)	2012-2015	417.2%
HCI (HCI)	2012-2015	404.87%
TAL Education (TAL)	2012-2015	391.51%

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