Quantum Computing (QUBT) had the best day in Q1 2015 by a mid-cap US stock on February 18, 2015, returning 200%.

ASSET	DATE	% RETURN
Quantum Computing (QUBT)	2/18/2015	200%
Quantum Computing (QUBT)	3/2/2015	200%
Quantum Computing (QUBT)	2/23/2015	100%
Soleno Therapeutics (SLNO)	2/18/2015	92.88%
Soleno Therapeutics (SLNO)	2/17/2015	47.27%
BlackBerry (BB)	1/14/2015	31.06%
Centrus Energy (LEU)	3/23/2015	30%
Stride (LRN)	1/29/2015	28.34%
Soleno Therapeutics (SLNO)	3/11/2015	28%
Soleno Therapeutics (SLNO)	2/2/2015	25.19%
Soleno Therapeutics (SLNO)	2/23/2015	25%
Comstock Resources (CRK)	2/2/2015	22.82%
Skyline (SKY)	1/26/2015	22.48%
Halozyme Therapeutics (HALO)	1/7/2015	22.15%
Indivior PLC Ordinary Shares (INDV)	3/25/2015	21.8%
Repligen (RGEN)	1/8/2015	21.63%
Forward Industries (FORD)	3/26/2015	20.79%
ACADIA Pharmaceuticals (ACAD)	3/10/2015	18.68%
PTC Therapeutics (PTCT)	2/23/2015	18.35%
Transportadora de Gas del Sur SA ADR (TGS)	3/11/2015	17.95%
Repligen (RGEN)	3/13/2015	17.93%
Corcept Therapeutics (CORT)	3/18/2015	17.57%
Uranium Energy (UEC)	1/26/2015	17.09%
Soleno Therapeutics (SLNO)	3/4/2015	16.67%
Centrus Energy (LEU)	3/12/2015	16.2%

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