Amneal Pharmaceuticals, Inc Class A Common Stock (AMRX) had the highest return between 2008 and 2012 by a mid-cap US stock, returning 719.6%.

ASSET	YEARS	% RETURN
Amneal Pharmaceuticals, Inc Class A Common Stock (AMRX)	2008-2012	719.6%
Sandstorm Gold Ltd N (SAND)	2008-2012	539.57%
NewMarket (NEU)	2008-2012	464.34%
Bausch Health Companies (BHC)	2008-2012	459.54%
Cirrus Logic (CRUS)	2008-2012	450.76%
Dorman Products (DORM)	2008-2012	427.68%
Dillards (DDS)	2008-2012	399.17%
Credit Acceptance (CACC)	2008-2012	384.88%
Trex (TREX)	2008-2012	330.91%
Polaris Industries (PII)	2008-2012	312.29%
LivaNova (LIVN)	2008-2012	299.16%
Beacon Roofing Supply (BECN)	2008-2012	296.66%
Axos Financial (AX)	2008-2012	289.51%
Bath & Body Works (BBWI)	2008-2012	286.28%
Main Street Capital (MAIN)	2008-2012	261.96%
Boston Beer (SAM)	2008-2012	258.25%
Jazz Pharmaceuticals (JAZZ)	2008-2012	256.9%
Generac (GNRC)	2008-2012	251.61%
Alaska Air (ALK)	2008-2012	247.22%
Maximus (MMS)	2008-2012	244.37%
FirstCash (FCFS)	2008-2012	238.01%
IPG Photonics (IPGP)	2008-2012	236.67%
CommVault Systems (CVLT)	2008-2012	232.52%
EXACT Sciences (EXAS)	2008-2012	225.85%
Nova (NVMI)	2008-2012	220.48%

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