First Majestic Silver (AG) had the highest return between 2001 and 2004 by a mid-cap US stock, returning 7,272.9%.

ASSET	YEARS	% RETURN
First Majestic Silver (AG)	2001-2004	7,272.89%
Urban Outfitters (URBN)	2001-2004	2,229.24%
FirstCash (FCFS)	2001-2004	1,656.44%
Dorman Products (DORM)	2001-2004	1,189.62%
Bank Ozk (OZK)	2001-2004	1,149.1%
Boyd Gaming (BYD)	2001-2004	1,095.26%
Penn National Gaming (PENN)	2001-2004	1,081.5%
Rush Enterprises B (RUSHB)	2001-2004	977.03%
Hecla Mining (HL)	2001-2004	936.42%
Darling Ingredients (DAR)	2001-2004	896.57%
BRF SA ADR (BRFS)	2001-2004	804.05%
SSR Mining (SSRM)	2001-2004	789.2%
Middleby (MIDD)	2001-2004	758.12%
CarMax (KMX)	2001-2004	688.55%
Frontline (FRO)	2001-2004	686.26%
Light & Wonder (LNW)	2001-2004	684.21%
Option Care Health (OPCH)	2001-2004	682.77%
Amedisys (AMED)	2001-2004	673.5%
Thor Industries (THO)	2001-2004	660.49%
Merit Medical Systems (MMSI)	2001-2004	654.57%
Gerdau SA ADR (GGB)	2001-2004	600.73%
MSA Safety (MSA)	2001-2004	599.91%
Pinnacle Financial Partners (PNFP)	2001-2004	596.04%
Enerpac Tool (EPAC)	2001-2004	596.04%
Graphic Packaging (GPK)	2001-2004	577.64%

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