Quantum Computing (QUBT) had the highest return between 2015 and 2018 by a mid-cap US stock, returning 1,400%.

ASSET	YEARS	% RETURN
Quantum Computing (QUBT)	2015-2018	1,400%
Burford Capital (BUR)	2015-2018	881.97%
Caesars Entertainment (CZR)	2015-2018	754.01%
Sarepta Therapeutics (SRPT)	2015-2018	648.49%
Casella Waste Systems (CWST)	2015-2018	598.28%
TAL Education (TAL)	2015-2018	471.59%
NexGen Energy (NXE)	2015-2018	459.75%
Ringcentral (RNG)	2015-2018	449.97%
Appfolio (APPF)	2015-2018	387.41%
Galapagos NV ADR (GLPG)	2015-2018	383.96%
Transportadora de Gas del Sur SA ADR (TGS)	2015-2018	356.82%
Corcept Therapeutics (CORT)	2015-2018	343.85%
Alpha Metallurgical Resources (AMR)	2015-2018	343.49%
Wingstop (WING)	2015-2018	327.44%
Wix (WIX)	2015-2018	326.13%
Novanta (NOVT)	2015-2018	325.39%
Planet Fitness (PLNT)	2015-2018	320.61%
Weibo (WB)	2015-2018	308.89%
Masimo (MASI)	2015-2018	305.17%
Supernus Pharmaceuticals (SUPN)	2015-2018	296.89%
Skyline (SKY)	2015-2018	282.78%
Align Technology (ALGN)	2015-2018	265.88%
Globant (GLOB)	2015-2018	264.53%
Mercury Systems (MRCY)	2015-2018	250.08%
SkyWest (SKYW)	2015-2018	242.79%

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