Vipshop (VIPS) had the highest return between 2013 and 2015 by a mid-cap US stock, returning 748.8%.

ASSET	YEARS	% RETURN
Vipshop (VIPS)	2013-2015	748.82%
ACADIA Pharmaceuticals (ACAD)	2013-2015	645.82%
Celsius (CELH)	2013-2015	593.14%
BioCryst Pharmaceuticals (BCRX)	2013-2015	549.06%
Pampa Energia SA ADR (PAM)	2013-2015	490.52%
Travere Therapeutics (TVTX)	2013-2015	467.35%
Catalyst Pharmaceuticals (CPRX)	2013-2015	463.22%
Nexstar Broadcasting (NXST)	2013-2015	463.13%
Ligand Pharmaceuticals (LGND)	2013-2015	409.49%
TAL Education (TAL)	2013-2015	379.08%
Ambarella (AMBA)	2013-2015	365.28%
Repligen (RGEN)	2013-2015	342.03%
JOYY (JOYY)	2013-2015	330.83%
Manhattan Associates (MANH)	2013-2015	326.7%
EPAM Systems (EPAM)	2013-2015	324.51%
Plug Power (PLUG)	2013-2015	322%
Alkermes (ALKS)	2013-2015	317.79%
Patrick Industries (PATK)	2013-2015	308.33%
Grupo Financiero Galicia SA ADR (GGAL)	2013-2015	296.72%
Transportadora de Gas del Sur SA ADR (TGS)	2013-2015	287.14%
Himax Technologies (HIMX)	2013-2015	283.55%
Alaska Air (ALK)	2013-2015	276.03%
BBVA Banco Frances SA ADR (BBAR)	2013-2015	265.43%
Pilgrims Pride (PPC)	2013-2015	256.04%
Amicus Therapeutics (FOLD)	2013-2015	243.97%

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