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	251.23%
Matador Resources (MTDR)	April 2020	206.09%
NGL Energy Partners (NGL)	April 2020	156.35%
Sasol (SSL)	April 2020	145.13%
Mesoblast (MESO)	April 2020	135.02%
Chord Energy (CHRD)	April 2020	118.77%
Aya Gold & Silver Inc Common Shares (AYA)	April 2020	116.92%
Delek Logistics Partners (DKL)	April 2020	107.08%
CNX Resources (CNX)	April 2020	104.24%
Talos Energy (TALO)	April 2020	102.31%
BioCryst Pharmaceuticals (BCRX)	April 2020	100.51%
Uranium Energy (UEC)	April 2020	100%
Murphy Oil (MUR)	April 2020	100%
Garrett Motion (GTX)	April 2020	96.76%
Arcus Biosciences (RCUS)	April 2020	91.38%
Liberty Energy (LBRT)	April 2020	90.32%
Calumet Specialty Products Partners (CLMT)	April 2020	84.76%
Bill Com (BILL)	April 2020	83.29%
Oceaneering International (OII)	April 2020	80.35%
Denison Mines (DNN)	April 2020	79.17%
AXT (AXTI)	April 2020	78.39%
Sibanye Gold Ltd ADR (SBSW)	April 2020	77.51%
Hess Midstream Partners (HESM)	April 2020	77.28%
Pitney Bowes (PBI)	April 2020	76.49%
Plains GP (PAGP)	April 2020	74.79%

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