Applied Digital (APLD) had the best month in 2018 by a mid-cap US stock in May 2018, returning 1,258.3%.

ASSET	MONTH	% RETURN
Applied Digital (APLD)	May 2018	1,258.33%
Viking Therapeutics (VKTX)	May 2018	141.89%
CleanSpark (CLSK)	September 2018	136%
CleanSpark (CLSK)	April 2018	121.67%
Acm Research (ACMR)	March 2018	83.31%
Applied Digital (APLD)	August 2018	80.23%
Energy of Minas Gerais Co DRC (CIG.C)	October 2018	79.52%
Twist Bioscience (TWST)	November 2018	77.58%
Liquidia Technologies (LQDA)	September 2018	76.97%
CleanSpark (CLSK)	March 2018	74.45%
Skyline (SKY)	January 2018	72.98%
Companhia Energetica de Minas Gerais CEMIG Pref ADR (CIG)	October 2018	67.65%
Arrowhead Pharmaceuticals (ARWR)	January 2018	67.48%
Arrowhead Pharmaceuticals (ARWR)	May 2018	66.2%
Comstock Resources (CRK)	May 2018	65.95%
Glaukos (GKOS)	August 2018	64.87%
Crispr Therapeutics (CRSP)	January 2018	63.67%
Baytex Energy (BTE)	April 2018	63.14%
Xenon Pharmaceuticals (XENE)	June 2018	62.83%
Perpetua Resources (PPTA)	January 2018	62.16%
Syndax Pharmaceuticals (SNDX)	March 2018	59.53%
Northern Oil & Gas (NOG)	May 2018	59.41%
CleanSpark (CLSK)	February 2018	59%
VNET Group Inc DRC (VNET)	May 2018	58.13%
Victory Capital (VCTR)	November 2018	57.66%

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