SM Energy (SM) had the highest return in April 2020 by a mid-cap US stock, returning 251.2%.

ASSET	MONTH	% RETURN
SM Energy (SM)	April 2020	251.24%
APA (APA)	April 2020	224.72%
Matador Resources (MTDR)	April 2020	206.08%
Antero Midstream Partners (AM)	April 2020	148.23%
Sasol (SSL)	April 2020	145.13%
Ovintiv (OVV)	April 2020	136.19%
Delek Logistics Partners (DKL)	April 2020	107.08%
Brinker International (EAT)	April 2020	106.93%
CNX Resources (CNX)	April 2020	104.24%
Murphy Oil (MUR)	April 2020	100%
Uranium Energy (UEC)	April 2020	99.28%
Garrett Motion (GTX)	April 2020	96.75%
Arcus Biosciences (RCUS)	April 2020	91.38%
Liberty Oilfield Services (LBRT)	April 2020	90.32%
BRP (DOOO)	April 2020	88.71%
Surgery Partners (SGRY)	April 2020	88.5%
Bill Com (BILL)	April 2020	83.29%
Oceaneering International (OII)	April 2020	80.35%
Denison Mines (DNN)	April 2020	79.17%
Etsy (ETSY)	April 2020	77.71%
Hess Midstream Partners (HESM)	April 2020	77.28%
Galaxy Digital (GLXY)	April 2020	76.08%
Plains GP (PAGP)	April 2020	74.79%
Fluor (FLR)	April 2020	74.37%
Spyre Therapeutics (SYRE)	April 2020	73.05%

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