QXO (QXO) had the highest return in Q1 2015 by a mid-cap US stock, returning 1,330.5%.

ASSET	QUARTER	% RETURN
QXO (QXO)	Q1 2015	1,330.5%
Soleno Therapeutics (SLNO)	Q1 2015	344.44%
Celsius (CELH)	Q1 2015	149.97%
Corcept Therapeutics (CORT)	Q1 2015	86.05%
Pampa Energia SA ADR (PAM)	Q1 2015	75.47%
Applied Blockchain (APLD)	Q1 2015	70.1%
Universal Display (OLED)	Q1 2015	67.14%
Wayfair (W)	Q1 2015	60.68%
Insperity (NSP)	Q1 2015	54.06%
Repligen (RGEN)	Q1 2015	53.1%
Vipshop (VIPS)	Q1 2015	52.86%
Transportadora de Gas del Sur SA ADR (TGS)	Q1 2015	52.72%
BBVA Banco Frances SA ADR (BBAR)	Q1 2015	48.83%
Iovance Biotherapeutics (IOVA)	Q1 2015	48.59%
Amneal Pharmaceuticals, Inc Class A Common Stock (AMRX)	Q1 2015	46.56%
Halozyme Therapeutics (HALO)	Q1 2015	46.16%
Grupo Financiero Galicia SA ADR (GGAL)	Q1 2015	46.06%
Ambarella (AMBA)	Q1 2015	45.6%
Catalyst Pharmaceuticals (CPRX)	Q1 2015	45.3%
Supernus Pharmaceuticals (SUPN)	Q1 2015	44.44%
Stonex (SNEX)	Q1 2015	44.04%
Ligand Pharmaceuticals (LGND)	Q1 2015	43.67%
OneMain (OMF)	Q1 2015	42.77%
Credit Acceptance (CACC)	Q1 2015	42.72%
Patrick Industries (PATK)	Q1 2015	41.36%

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