Weatherford International (WFRD) had the highest return between 2015 and 2019 by a mid-cap US stock, returning 356,032.5%.

ASSET	YEARS	% RETURN
Weatherford International (WFRD)	2015-2019	356,032.47%
Quantum Computing (QUBT)	2015-2019	1,400%
Caesars Entertainment (CZR)	2015-2019	1,306.6%
Axsome Therapeutics (AXSM)	2015-2019	1,048.44%
Casella Waste Systems (CWST)	2015-2019	1,028.19%
Ringcentral (RNG)	2015-2019	1,025.22%
Galapagos NV ADR (GLPG)	2015-2019	991.11%
TAL Education (TAL)	2015-2019	932.63%
Appfolio (APPF)	2015-2019	804.94%
Sarepta Therapeutics (SRPT)	2015-2019	785.05%
Skyline (SKY)	2015-2019	726.02%
Arrowhead Pharmaceuticals (ARWR)	2015-2019	722.7%
Universal Display (OLED)	2015-2019	640.81%
Globant (GLOB)	2015-2019	586.41%
New Oriental Education & Technology (EDU)	2015-2019	503.54%
Novanta (NOVT)	2015-2019	497.16%
Masimo (MASI)	2015-2019	496.45%
Planet Fitness (PLNT)	2015-2019	485.81%
Wix (WIX)	2015-2019	477.26%
Wingstop (WING)	2015-2019	476.98%
MarketAxess (MKTX)	2015-2019	445.61%
Krystal Biotech (KRYS)	2015-2019	427.43%
GDS (GDS)	2015-2019	415.8%
Mercury Systems (MRCY)	2015-2019	411.62%
SkyWest (SKYW)	2015-2019	402.32%

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