Vipshop (VIPS) had the highest return between 2011 and 2014 by a mid-cap US stock, returning 3,156.6%.

ASSET	YEARS	% RETURN
Vipshop (VIPS)	2011-2014	3,156.64%
Par Pacific (PARR)	2011-2014	2,608.33%
ACADIA Pharmaceuticals (ACAD)	2011-2014	2,568.07%
Patrick Industries (PATK)	2011-2014	2,088.17%
Travere Therapeutics (TVTX)	2011-2014	1,260%
Nexstar Broadcasting (NXST)	2011-2014	791.65%
Ambarella (AMBA)	2011-2014	655.89%
HCI (HCI)	2011-2014	529.27%
Lithia Motors (LAD)	2011-2014	525.87%
JOYY (JOYY)	2011-2014	493.72%
Ligand Pharmaceuticals (LGND)	2011-2014	493.19%
Silicon Motion Technology (SIMO)	2011-2014	491.22%
Boyd Group Services (BGSI)	2011-2014	480.76%
Addus HomeCare (ADUS)	2011-2014	479.24%
Astronics (ATRO)	2011-2014	444.54%
American Airlines (AAL)	2011-2014	433.15%
Manhattan Associates (MANH)	2011-2014	429.52%
Bausch Health Companies (BHC)	2011-2014	403.38%
Under Armour Inc A (UAA)	2011-2014	394%
Axos Financial (AX)	2011-2014	389.37%
Alkermes (ALKS)	2011-2014	375.32%
CleanSpark (CLSK)	2011-2014	374.36%
Alaska Air (ALK)	2011-2014	322.67%
Repligen (RGEN)	2011-2014	321.28%
Avis Budget (CAR)	2011-2014	320.61%

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