Arrowhead Pharmaceuticals (ARWR) had the highest return between 2017 and 2019 by a mid-cap US stock, returning 3,966%.

ASSET	YEARS	% RETURN
Arrowhead Pharmaceuticals (ARWR)	2017-2019	3,966.03%
Enphase Energy (ENPH)	2017-2019	2,461.76%
Quantum Computing (QUBT)	2017-2019	1,400%
Axsome Therapeutics (AXSM)	2017-2019	1,376.57%
Aura Minerals Inc Common Shares (AUGO)	2017-2019	1,329.98%
Teekay Tankers (TNK)	2017-2019	1,049.81%
Aurinia Pharmaceuticals (AUPH)	2017-2019	846.73%
Ringcentral (RNG)	2017-2019	703.19%
RH (RH)	2017-2019	593.18%
Viking Therapeutics (VKTX)	2017-2019	568.33%
Crocs (CROX)	2017-2019	501.87%
Freshpet (FRPT)	2017-2019	479.31%
Paycom Software (PAYC)	2017-2019	473.07%
Vericel Corp Ord (VCEL)	2017-2019	470.49%
GDS (GDS)	2017-2019	464.33%
Weatherford International (WFRD)	2017-2019	445.9%
Krystal Biotech (KRYS)	2017-2019	427.43%
Match (MTCH)	2017-2019	400.7%
Ero Copper (ERO)	2017-2019	369.75%
Appfolio (APPF)	2017-2019	359.08%
Sarepta Therapeutics (SRPT)	2017-2019	355.17%
PTC Therapeutics (PTCT)	2017-2019	334.27%
Novanta (NOVT)	2017-2019	316.19%
TAL Education (TAL)	2017-2019	308.98%
Aris Mining (ARMN)	2017-2019	305.71%

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