NexGen Energy (NXE) had the highest return between 2015 and 2016 by a mid-cap US stock, returning 441.2%.

ASSET	YEARS	% RETURN
NexGen Energy (NXE)	2015-2016	441.19%
Alpha Metallurgical Resources (AMR)	2015-2016	320.74%
Caesars Entertainment (CZR)	2015-2016	299.76%
DXC Technology (DXC)	2015-2016	294.3%
DRDGOLD Limited ADR (DRD)	2015-2016	291.56%
Skyline (SKY)	2015-2016	285.75%
Pampa Energia SA ADR (PAM)	2015-2016	245.68%
Galapagos NV ADR (GLPG)	2015-2016	238.63%
Burford Capital (BUR)	2015-2016	213.82%
Casella Waste Systems (CWST)	2015-2016	204.17%
Supernus Pharmaceuticals (SUPN)	2015-2016	201.67%
Weibo (WB)	2015-2016	184.12%
SkyWest (SKYW)	2015-2016	176.63%
JBTMarel (JBTM)	2015-2016	163.25%
Transportadora de Gas del Sur SA ADR (TGS)	2015-2016	160.74%
Masimo (MASI)	2015-2016	154.34%
TAL Education (TAL)	2015-2016	149.47%
Ingevity (NGVT)	2015-2016	149.36%
IES (IESC)	2015-2016	147.74%
Corcept Therapeutics (CORT)	2015-2016	141.2%
Cirrus Logic (CRUS)	2015-2016	139.68%
National Beverage (FIZZ)	2015-2016	132.35%
Installed Building Products (IBP)	2015-2016	131.24%
Towne Bank (TOWN)	2015-2016	130.45%
ServisFirst Bancshares (SFBS)	2015-2016	130.43%

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