CarMax (KMX) had the highest return between 2000 and 2003 by a mid-cap US stock, returning 1,274.7%.

ASSET	YEARS	% RETURN
CarMax (KMX)	2000-2003	1,274.67%
Meritage (MTH)	2000-2003	1,133.63%
Cal-Maine Foods (CALM)	2000-2003	964.7%
FTI Consulting (FCN)	2000-2003	951.66%
Companhia Siderurgica Nacional ADR (SID)	2000-2003	941.8%
Hudbay Minerals (HBM)	2000-2003	825.08%
SSR Mining (SSRM)	2000-2003	792.79%
Perdoceo Education (PRDO)	2000-2003	747.38%
Merit Medical Systems (MMSI)	2000-2003	724.44%
Middleby (MIDD)	2000-2003	645.93%
Brightstar Lottery (BRSL)	2000-2003	607.39%
RadNet (RDNT)	2000-2003	558.82%
Comstock Resources (CRK)	2000-2003	557.02%
DaVita HealthCare Partners (DVA)	2000-2003	477.78%
Inter Parfums (IPAR)	2000-2003	471.72%
Denison Mines (DNN)	2000-2003	471.36%
Strategic Education (STRA)	2000-2003	462.78%
Bank Ozk (OZK)	2000-2003	435.36%
Primo Brands (PRMB)	2000-2003	433.53%
Penn National Gaming (PENN)	2000-2003	415.55%
M/I Homes (MHO)	2000-2003	413.3%
Hecla Mining (HL)	2000-2003	410.17%
Light & Wonder (LNW)	2000-2003	402.81%
Bio-Rad Laboratories (BIO)	2000-2003	393.43%
Henry Schein (HSIC)	2000-2003	384.87%

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