AutoNation (AN) had the highest return between 1994 and 1996 by a mid-cap US stock, returning 2,656.4%.

ASSET	YEARS	% RETURN
AutoNation (AN)	1994-1996	2,656.42%
Bausch Health Companies (BHC)	1994-1996	1,181.32%
Viavi Solutions (VIAV)	1994-1996	1,135.34%
MasTec (MTZ)	1994-1996	802.1%
CBIZ (CBZ)	1994-1996	646.15%
Semtech (SMTC)	1994-1996	620.98%
Hexcel (HXL)	1994-1996	456.07%
Federal Agricultural Mortgage (AGM.A)	1994-1996	454.99%
Robert Half International (RHI)	1994-1996	422.5%
Freedom (FRHC)	1994-1996	350%
Onto Innovation (ONTO)	1994-1996	347.06%
Bio-Rad Laboratories (BIO)	1994-1996	339.02%
Sanmina (SANM)	1994-1996	338.83%
Belden (BDC)	1994-1996	336.76%
Penn National Gaming (PENN)	1994-1996	327.57%
Comstock Resources (CRK)	1994-1996	316%
Weatherford International (WFRD)	1994-1996	307%
Transocean (RIG)	1994-1996	298.73%
Patterson-UTI Energy (PTEN)	1994-1996	296.19%
ACI Worldwide (ACIW)	1994-1996	272.02%
CACI International (CACI)	1994-1996	257.45%
Adtalem Global Education (ATGE)	1994-1996	235.72%
WPP PLC ADR (WPP)	1994-1996	227.42%
National Beverage (FIZZ)	1994-1996	219.96%
Alkermes (ALKS)	1994-1996	210%

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