Riot Blockchain (RIOT) had the highest return in December 2017 by a mid-cap US stock, returning 91.2%.

ASSET	MONTH	% RETURN
Riot Blockchain (RIOT)	December 2017	91.25%
Apellis Pharmaceuticals (APLS)	December 2017	56.45%
Century Aluminum (CENX)	December 2017	48.23%
Aura Minerals Inc Common Shares (AUGO)	December 2017	46.36%
Vipshop (VIPS)	December 2017	43.45%
Appian (APPN)	December 2017	41.48%
CleanSpark (CLSK)	December 2017	39.43%
Comstock Resources (CRK)	December 2017	37.56%
Spyre Therapeutics (SYRE)	December 2017	33.91%
Viking Therapeutics (VKTX)	December 2017	33.11%
Uranium Energy (UEC)	December 2017	33.08%
Solaris Energy Infrastructure (SEI)	December 2017	31.59%
Viridian Therapeutics (VRDN)	December 2017	30.7%
ADEIA (ADEA)	December 2017	26.42%
Soleno Therapeutics (SLNO)	December 2017	25.87%
Eldorado Gold (EGO)	December 2017	25.44%
Fortuna Silver Mines (FSM)	December 2017	24.88%
National Vision (EYE)	December 2017	24.53%
Weatherford International (WFRD)	December 2017	24.48%
Crispr Therapeutics (CRSP)	December 2017	23.25%
Bausch Health Companies (BHC)	December 2017	23.03%
B2Gold (BTG)	December 2017	21.09%
Floor & Decor (FND)	December 2017	20.58%
Golar LNG (GLNG)	December 2017	20.53%
Ero Copper (ERO)	December 2017	19.24%

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