Green Brick Partners (GRBK) had the best quarter in 2011 by a mid-cap US stock in Q4 2011, returning 240%.

ASSET	QUARTER	% RETURN
Green Brick Partners (GRBK)	Q4 2011	240.03%
Patrick Industries (PATK)	Q4 2011	121.6%
MGIC Investment (MTG)	Q4 2011	102.72%
Silicon Motion Technology (SIMO)	Q1 2011	86.17%
IPG Photonics (IPGP)	Q1 2011	81.21%
Silicon Motion Technology (SIMO)	Q4 2011	81.08%
Universal Display (OLED)	Q1 2011	76.69%
Bausch Health Companies (BHC)	Q1 2011	75.2%
Gibraltar Industries (ROCK)	Q4 2011	74.28%
Sapiens International (SPNS)	Q1 2011	72.72%
Cemex SAB de CV ADR (CX)	Q4 2011	71.67%
Astrana Health (ASTH)	Q1 2011	71.43%
Golar LNG (GLNG)	Q1 2011	71.16%
R1 RCM (RCM)	Q1 2011	70.2%
Catalyst Pharmaceuticals (CPRX)	Q2 2011	69.37%
Melco Resorts & Entertainment (MLCO)	Q2 2011	64.77%
M/I Homes (MHO)	Q4 2011	61.07%
SPS Commerce (SPSC)	Q4 2011	60.88%
ASGN (ASGN)	Q4 2011	60.63%
Chart Industries (GTLS)	Q1 2011	60.51%
Jazz Pharmaceuticals (JAZZ)	Q1 2011	60.21%
WESCO International (WCC)	Q4 2011	59.91%
Louisiana-Pacific (LPX)	Q4 2011	59.17%
Halozyme Therapeutics (HALO)	Q4 2011	57.19%
Eagle Materials (EXP)	Q4 2011	57.04%

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