Northern Oil & Gas (NOG) had the highest return between 2006 and 2008 by a mid-cap US stock, returning 852,476.9%.

ASSET	YEARS	% RETURN
Northern Oil & Gas (NOG)	2006-2008	852,476.92%
Weatherford International (WFRD)	2006-2008	7,300%
Riot Blockchain (RIOT)	2006-2008	517%
Argan (AGX)	2006-2008	445.01%
RadNet (RDNT)	2006-2008	440.32%
Cal-Maine Foods (CALM)	2006-2008	339.12%
MAG Silver (MAG)	2006-2008	233.33%
Adtalem Global Education (ATGE)	2006-2008	212.44%
ICL Israel Chemicals (ICL)	2006-2008	210.6%
Halozyme Therapeutics (HALO)	2006-2008	207.69%
AZZ (AZZ)	2006-2008	179.66%
PriceSmart (PSMT)	2006-2008	152.57%
New Oriental Education & Technology (EDU)	2006-2008	149.59%
Strategic Education (STRA)	2006-2008	135.7%
Huron Consulting (HURN)	2006-2008	135.49%
Perrigo (PRGO)	2006-2008	122.42%
Companhia Siderurgica Nacional ADR (SID)	2006-2008	121%
Exponent (EXPO)	2006-2008	113.71%
Open Text (OTEX)	2006-2008	113.69%
American Superconductor (AMSC)	2006-2008	99.15%
Dolby Laboratories (DLB)	2006-2008	92.93%
Balchem (BCPC)	2006-2008	90.59%
VSE (VSEC)	2006-2008	87.97%
Grand Canyon Education (LOPE)	2006-2008	87.8%
Valmont Industries (VMI)	2006-2008	86.2%

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