Kinetik (KNTK) had the highest return between 2015 and 2018 by a mid-cap US stock, returning 7,194,614.3%.

ASSET	YEARS	% RETURN
Kinetik (KNTK)	2015-2018	7,194,614.29%
CleanSpark (CLSK)	2015-2018	1,131,483.33%
Aura Minerals Inc Common Shares (AUGO)	2015-2018	1,875.3%
Quantum Computing (QUBT)	2015-2018	1,400%
Caesars Entertainment (CZR)	2015-2018	754.01%
Sarepta Therapeutics (SRPT)	2015-2018	648.49%
Casella Waste Systems (CWST)	2015-2018	598.28%
Celsius (CELH)	2015-2018	593.88%
NexGen Energy (NXE)	2015-2018	493.33%
TAL Education (TAL)	2015-2018	471.59%
Axogen (AXGN)	2015-2018	467.5%
Ringcentral (RNG)	2015-2018	449.97%
Appfolio (APPF)	2015-2018	387.41%
Paycom Software (PAYC)	2015-2018	365.77%
Transportadora de Gas del Sur SA ADR (TGS)	2015-2018	356.82%
Wayfair (W)	2015-2018	350.63%
Corcept Therapeutics (CORT)	2015-2018	343.85%
Alpha Metallurgical Resources (AMR)	2015-2018	343.49%
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%
AnaptysBio (ANAB)	2015-2018	298.69%
Supernus Pharmaceuticals (SUPN)	2015-2018	296.89%

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