Aris Mining (ARMN) had the highest return between 2008 and 2011 by a mid-cap US stock, returning 32,677%.

ASSET	YEARS	% RETURN
Aris Mining (ARMN)	2008-2011	32,676.97%
Weatherford International (WFRD)	2008-2011	9,900%
Mesoblast (MESO)	2008-2011	979.72%
Amneal Pharmaceuticals, Inc Class A Common Stock (AMRX)	2008-2011	706.8%
Bausch Health Companies (BHC)	2008-2011	337.09%
Credit Acceptance (CACC)	2008-2011	292.37%
Steven Madden (SHOO)	2008-2011	288.71%
NewMarket (NEU)	2008-2011	282.31%
Northern Oil & Gas (NOG)	2008-2011	268.92%
First Majestic Silver (AG)	2008-2011	250.1%
Sandstorm Gold Ltd N (SAND)	2008-2011	224.66%
Alaska Air (ALK)	2008-2011	202.54%
Cirrus Logic (CRUS)	2008-2011	201.33%
Bath & Body Works (BBWI)	2008-2011	199.09%
Nova (NVMI)	2008-2011	195.98%
Boston Beer (SAM)	2008-2011	189.26%
Perrigo (PRGO)	2008-2011	187.45%
Sapiens International (SPNS)	2008-2011	178.53%
Balchem (BCPC)	2008-2011	177.19%
Hawkins (HWKN)	2008-2011	173.35%
Golar LNG (GLNG)	2008-2011	172.89%
Alliance Resource Partners (ARLP)	2008-2011	169.83%
Polaris Industries (PII)	2008-2011	168.93%
Trex (TREX)	2008-2011	165.17%
Dorman Products (DORM)	2008-2011	163.79%

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