Frontline (FRO) had the highest return between 2004 and 2006 by a mid-cap US stock, returning 1,428.4%.

ASSET	YEARS	% RETURN
Frontline (FRO)	2004-2006	1,428.38%
Companhia Energetica de Minas Gerais CEMIG Pref ADR (CIG)	2004-2006	1,148.15%
BRF SA ADR (BRFS)	2004-2006	1,119.87%
HF Sinclair (DINO)	2004-2006	667.02%
Allegheny Technologies (ATI)	2004-2006	609.24%
WESCO International (WCC)	2004-2006	563.02%
Seaboard (SEB)	2004-2006	532.41%
Icahn Enterprises (IEP)	2004-2006	411.79%
Hub (HUBG)	2004-2006	402.51%
Sterling Construction (STRL)	2004-2006	386.8%
Stonex (SNEX)	2004-2006	370.64%
Terex (TEX)	2004-2006	351.61%
Grupo Simec SAB de CV ADR (SIM)	2004-2006	350.47%
Range Resources (RRC)	2004-2006	338.39%
United States Lime & Minerals (USLM)	2004-2006	330.72%
Gerdau SA ADR (GGB)	2004-2006	322.11%
RadNet (RDNT)	2004-2006	320%
Skechers (SKX)	2004-2006	308.71%
Woori Financial (WF)	2004-2006	306.41%
Uranium Energy (UEC)	2004-2006	298.06%
Cleveland-Cliffs (CLF)	2004-2006	288.75%
AZZ (AZZ)	2004-2006	283.52%
BlackBerry (BB)	2004-2006	280.52%
Mueller Industries (MLI)	2004-2006	263.74%
Penn National Gaming (PENN)	2004-2006	260.04%

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