Northern Oil & Gas (NOG) had the highest return between 2006 and 2007 by a mid-cap US stock, returning 2,241,830.8%.

ASSET	YEARS	% RETURN
Northern Oil & Gas (NOG)	2006-2007	2,241,830.77%
RadNet (RDNT)	2006-2007	1,537.1%
Riot Blockchain (RIOT)	2006-2007	772%
Argan (AGX)	2006-2007	567.52%
Mosaic (MOS)	2006-2007	544.4%
BlackBerry (BB)	2006-2007	410.81%
Companhia Siderurgica Nacional ADR (SID)	2006-2007	401.28%
Millicom International Cellular (TIGO)	2006-2007	328.25%
Cogent Communications (CCOI)	2006-2007	322.65%
AGCO (AGCO)	2006-2007	305.85%
ICL Israel Chemicals (ICL)	2006-2007	292.46%
Halozyme Therapeutics (HALO)	2006-2007	290.66%
Cal-Maine Foods (CALM)	2006-2007	288.07%
Hudbay Minerals (HBM)	2006-2007	277.35%
New Oriental Education & Technology (EDU)	2006-2007	266.32%
PriceSmart (PSMT)	2006-2007	260.68%
Uranium Energy (UEC)	2006-2007	253.06%
Icahn Enterprises (IEP)	2006-2007	241.05%
American Superconductor (AMSC)	2006-2007	233.82%
Huron Consulting (HURN)	2006-2007	231.54%
AZZ (AZZ)	2006-2007	215.87%
Seabridge Gold (SA)	2006-2007	200.41%
Dolby Laboratories (DLB)	2006-2007	192.81%
Darling Ingredients (DAR)	2006-2007	191.18%
Stonex (SNEX)	2006-2007	188.66%

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