Weatherford International (WFRD) had the highest return on December 31, 2019 by a mid-cap US stock, returning 9.6%.

ASSET	DATE	% RETURN
Weatherford International (WFRD)	12/31/2019	9.61%
VNET Group Inc DRC (VNET)	12/31/2019	8.21%
Comstock Resources (CRK)	12/31/2019	8.15%
Celcuity LLC (CELC)	12/31/2019	7.47%
Amneal Pharmaceuticals, Inc Class A Common Stock (AMRX)	12/31/2019	7.11%
Taseko Mines (TGB)	12/31/2019	6.67%
Crinetics Pharmaceuticals (CRNX)	12/31/2019	6.31%
United Natural Foods (UNFI)	12/31/2019	6.18%
American Bitcoin (ABTC)	12/31/2019	5.24%
Northern Oil & Gas (NOG)	12/31/2019	4.93%
Centrus Energy (LEU)	12/31/2019	4.88%
StoneCo (STNE)	12/31/2019	4.67%
Golar LNG (GLNG)	12/31/2019	4.48%
Adaptive Biotechnologies (ADPT)	12/31/2019	4.29%
Antero Midstream Partners (AM)	12/31/2019	4.26%
Apellis Pharmaceuticals (APLS)	12/31/2019	4.22%
Palomar (PLMR)	12/31/2019	4.19%
Upwork (UPWK)	12/31/2019	4.1%
Zai Lab (ZLAB)	12/31/2019	3.98%
Turning Point Brands (TPB)	12/31/2019	3.89%
Enphase Energy (ENPH)	12/31/2019	3.86%
CNX Resources (CNX)	12/31/2019	3.63%
Plug Power (PLUG)	12/31/2019	3.61%
ELF Beauty (ELF)	12/31/2019	3.6%
Immunovant (IMVT)	12/31/2019	3.52%

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