First Majestic Silver (AG) had the highest return between 2000 and 2005 by a mid-cap US stock, returning 9,373.8%.

ASSET	YEARS	% RETURN
First Majestic Silver (AG)	2000-2005	9,373.78%
Meritage (MTH)	2000-2005	2,241.13%
Companhia Siderurgica Nacional ADR (SID)	2000-2005	1,940.78%
HF Sinclair (DINO)	2000-2005	1,907.31%
Denison Mines (DNN)	2000-2005	1,665.05%
Middleby (MIDD)	2000-2005	1,504.53%
Penn National Gaming (PENN)	2000-2005	1,369.51%
FTI Consulting (FCN)	2000-2005	1,134.81%
CarMax (KMX)	2000-2005	1,130.22%
SSR Mining (SSRM)	2000-2005	1,127.19%
DaVita HealthCare Partners (DVA)	2000-2005	1,025.33%
Comstock Resources (CRK)	2000-2005	938.64%
Gildan Activewear (GIL)	2000-2005	852.17%
Alliance Resource Partners (ARLP)	2000-2005	851.72%
Frontline (FRO)	2000-2005	838.44%
Bank Ozk (OZK)	2000-2005	796.85%
Boyd Gaming (BYD)	2000-2005	764.84%
Hudbay Minerals (HBM)	2000-2005	737.57%
Light & Wonder (LNW)	2000-2005	708.3%
CNX Resources (CNX)	2000-2005	696.92%
Methanex (MEOH)	2000-2005	693.07%
Seaboard (SEB)	2000-2005	687.83%
Landstar System (LSTR)	2000-2005	657.79%
ESCO Technologies (ESE)	2000-2005	657.27%
Flowers Foods (FLO)	2000-2005	620.63%

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