Enphase Energy (ENPH) had the lowest return between 2015 and 2016 by a mid-cap US stock, returning -92.9%.

ASSET	YEARS	% RETURN
Enphase Energy (ENPH)	2015-2016	-92.94%
Tidewater (TDW)	2015-2016	-88.7%
Viking Therapeutics (VKTX)	2015-2016	-86%
Option Care Health (OPCH)	2015-2016	-85.16%
Star Bulk Carriers (SBLK)	2015-2016	-84.56%
Immunitybio (IBRX)	2015-2016	-84.54%
Navios Maritime Partners LP Unit (NMM)	2015-2016	-83.96%
POET Technologies (POET)	2015-2016	-81.25%
Celldex Therapeutics (CLDX)	2015-2016	-80.77%
Calumet Specialty Products Partners (CLMT)	2015-2016	-79.37%
PTC Therapeutics (PTCT)	2015-2016	-79.23%
Energy Fuels (UUUU)	2015-2016	-73.68%
Riot Platforms (RIOT)	2015-2016	-72.88%
Comstock Resources (CRK)	2015-2016	-70.99%
TG Therapeutics (TGTX)	2015-2016	-70.81%
Universal Technical Institute (UTI)	2015-2016	-69.55%
Almonty Industries Inc Common Shares (ALM)	2015-2016	-69.08%
Baytex Energy (BTE)	2015-2016	-69.05%
RH (RH)	2015-2016	-68.08%
Brookdale Senior Living (BKD)	2015-2016	-66.3%
Century Aluminum (CENX)	2015-2016	-65.09%
Catalyst Pharmaceuticals (CPRX)	2015-2016	-64.77%
Constellium (CSTM)	2015-2016	-64.31%
Xenon Pharmaceuticals (XENE)	2015-2016	-61.09%
Plug Power (PLUG)	2015-2016	-59.6%

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