Par Pacific (PARR) had the highest return between 2012 and 2013 by a mid-cap US stock, returning 3,616.7%.

ASSET	YEARS	% RETURN
Par Pacific (PARR)	2012-2013	3,616.67%
ACADIA Pharmaceuticals (ACAD)	2012-2013	2,171.82%
Vipshop (VIPS)	2012-2013	1,294.64%
HCI (HCI)	2012-2013	634.21%
Nexstar Broadcasting (NXST)	2012-2013	612.38%
Patrick Industries (PATK)	2012-2013	600.48%
Radian (RDN)	2012-2013	498.21%
Ambarella (AMBA)	2012-2013	405.07%
American Airlines (AAL)	2012-2013	385.58%
Acadia Healthcare (ACHC)	2012-2013	381.98%
Axos Financial (AX)	2012-2013	381.76%
JOYY (JOYY)	2012-2013	378.86%
Sarepta Therapeutics (SRPT)	2012-2013	349.67%
Ligand Pharmaceuticals (LGND)	2012-2013	338.33%
Melco Resorts & Entertainment (MLCO)	2012-2013	297.76%
Repligen (RGEN)	2012-2013	294.22%
Mueller Water Products (MWA)	2012-2013	290.48%
Western Alliance Bancorporation (WAL)	2012-2013	276.34%
Saia (SAIA)	2012-2013	271.81%
Travere Therapeutics (TVTX)	2012-2013	268.42%
Archrock (AROC)	2012-2013	265.76%
Avis Budget (CAR)	2012-2013	264.14%
Zillow (ZG)	2012-2013	263.24%
DXP Enterprises (DXPE)	2012-2013	245.32%
Grifols SA ADR (GRFS)	2012-2013	242.04%

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