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%
Quantum Computing (QUBT)	January 2018	650%
Quantum Computing (QUBT)	February 2018	242.86%
Viking Therapeutics (VKTX)	May 2018	141.89%
CleanSpark (CLSK)	September 2018	136%
Madrigal Pharmaceuticals (MDGL)	May 2018	134.08%
Quantum Computing (QUBT)	September 2018	130%
CleanSpark (CLSK)	April 2018	121.67%
Quantum Computing (QUBT)	July 2018	116.67%
Forward Industries (FORD)	January 2018	100.83%
Sharplink Gaming (SBET)	July 2018	85.71%
Acm Research (ACMR)	March 2018	83.31%
Energy of Minas Gerais Co DRC (CIG.C)	October 2018	79.51%
Applied Digital (APLD)	August 2018	75.94%
CleanSpark (CLSK)	March 2018	74.45%
Skyline (SKY)	January 2018	72.98%
Onto Innovation (ONTO)	May 2018	68.89%
Companhia Energetica de Minas Gerais CEMIG Pref ADR (CIG)	October 2018	67.64%
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%
Xenon Pharmaceuticals (XENE)	June 2018	62.83%
Perpetua Resources (PPTA)	January 2018	62.2%

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