Frontline (FRO) had the highest return between 2000 and 2009 by a mid-cap US stock, returning 18,582.8%.

ASSET	YEARS	% RETURN
Frontline (FRO)	2000-2009	18,582.8%
Companhia Siderurgica Nacional ADR (SID)	2000-2009	12,673.16%
Gerdau SA ADR (GGB)	2000-2009	2,420.17%
Cal-Maine Foods (CALM)	2000-2009	2,089.65%
Hudbay Minerals (HBM)	2000-2009	2,058.51%
CarMax (KMX)	2000-2009	2,055.56%
FTI Consulting (FCN)	2000-2009	2,022.22%
HF Sinclair (DINO)	2000-2009	1,758.26%
Middleby (MIDD)	2000-2009	1,718.59%
SSR Mining (SSRM)	2000-2009	1,649.58%
Sterling Construction (STRL)	2000-2009	1,431.2%
Balchem (BCPC)	2000-2009	1,390.47%
Alliance Resource Partners (ARLP)	2000-2009	1,333.21%
Comstock Resources (CRK)	2000-2009	1,281.11%
VSE (VSEC)	2000-2009	1,263.98%
Cleveland-Cliffs (CLF)	2000-2009	1,247.71%
DaVita HealthCare Partners (DVA)	2000-2009	1,205.33%
Penn National Gaming (PENN)	2000-2009	1,168.52%
CNX Resources (CNX)	2000-2009	1,159.04%
RadNet (RDNT)	2000-2009	1,100%
Strategic Education (STRA)	2000-2009	1,068.06%
ICL Israel Chemicals (ICL)	2000-2009	1,042.49%
Credit Acceptance (CACC)	2000-2009	1,022.67%
Itron (ITRI)	2000-2009	1,014.56%
Aura Minerals Inc Common Shares (AUGO)	2000-2009	1,007.2%

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