Par Pacific (PARR) had the highest return between 2011 and 2012 by a mid-cap US stock, returning 1,900%.

ASSET	YEARS	% RETURN
Par Pacific (PARR)	2011-2012	1,900%
Patrick Industries (PATK)	2011-2012	674.12%
ACADIA Pharmaceuticals (ACAD)	2011-2012	290.76%
Travere Therapeutics (TVTX)	2011-2012	272.22%
Delek US Energy (DK)	2011-2012	262.68%
Acadia Healthcare (ACHC)	2011-2012	243.38%
Silicon Motion Technology (SIMO)	2011-2012	227.65%
CVR Energy (CVI)	2011-2012	216.61%
Vipshop (VIPS)	2011-2012	197.32%
HCI (HCI)	2011-2012	185.88%
Lithia Motors (LAD)	2011-2012	166.24%
Golar LNG (GLNG)	2011-2012	165.06%
Melco Resorts & Entertainment (MLCO)	2011-2012	156.71%
Fortune Brands Innovations (FBIN)	2011-2012	145.55%
CommVault Systems (CVLT)	2011-2012	141.46%
NewMarket (NEU)	2011-2012	138.38%
SPS Commerce (SPSC)	2011-2012	133.23%
Dillard's (DDS)	2011-2012	132.46%
Ligand Pharmaceuticals (LGND)	2011-2012	131.21%
Stewart Information Services (STC)	2011-2012	125.02%
Boyd Group Services (BGSI)	2011-2012	124.91%
Polaris Industries (PII)	2011-2012	121.47%
IPG Photonics (IPGP)	2011-2012	111.54%
Eagle Materials (EXP)	2011-2012	111.13%
Argan (AGX)	2011-2012	108.84%

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