Sigma by a good margin is the sharper and better performing lens in 85mm f/1.4. Specifically the Art lens.
For the less expensive Nikon 85mm f/1.8 D and G models. The G model is slightly sharper, but the D is slightly in better light transmission. The G has more Chromatic aberrations over than the D.
For Sigma A 85mm f/1.4 versus Nikon 85mm f/1.4 D and G lenses. The Sigma again is the winner. This is for actual visual measurements, not af speed, build or anything like that.
Again the G is slightly sharper than the D model f/1.4 Nikons. Transmission the G slightly beats out the D. Distortion the D beats the G. Vignetting the D slightly beats the G. About the same for Chromatic Aberrations.
The Tamron 85mm f/1.8 Scores lower than the Sigma but higher than the Nikon 85mm f/1.4G. The Tamron has better sharpness, less transmission though, less distortion, same vignetting, slightly less chromatic aberrations.
This is from DXO's measurement charts and scores.
I have the 85mm f/1.8D lens and so far have not needed to replace it. With the sensors getting higher in resolution. I have been considering going to sharper lens lately though.
In all fairness: the Nikkor 85/1.8 AF and AF-D lenses are dreadful; loads of purple fringing on so,so many things. Really bad compared to the same-era f/1.4 AF-D model. Gave mine away to a friend in exchange for a vacation stay-over. The two 1.8 models AF, and AF-D, seem to be the same, and are bad 85mm primes, compared to the earlier Ai and Ai-S models, which were pretty good.
With higher test sharp sharpness often comes over-corrected spherical aberration, and AWFUL, harsh, or nervous, or hashy bokeh. This is where the Sigma and Tamron lenses fail and fall flat. The ART series has some of the ugliest bokeh one could ever hope to be saddled with...this is why the old 85,105,and 135 f/1.4, f/2, and f/2 Nikkor lenses of the 1990's were popular with people shooters, for so,so long; same reason the now-aging Canon 135/2-L is so amazing:
pretty bokeh and the right balance of imaging characteristics.
The lenses that test high on resolution and are new and designed by third-party makers are often lacking that right balance of imaging characteristics, IMO. Line pairs per millimeter reolved is pretty danged unimportant if every image has hash for backgrounds, and if OOF elements tend to
double-line and create annoying edges on objects.
It's easy for a Korean company like Samyang to build a lens that can separate and reveal fine lines on a test chart, but the lens itself has to offer smooth defocus, and decent bokeh, or it's rubbish for people pictures. The new Tamron 85mm f/1.8 with Vibration Control is exceptionally UGLY in the way it renders the out of focus zones on high-frequency detail like foliage, leaves, sticks, fencing, etc.. The new, cheap Korean 135mm lens that 'beats' say, the Canon 135/2-L on test charts: looks like rubbish, even though it "beats" Canon's 20-plus-year old,magical 135mm f/2 L USM lens on test charts.
Optical "flaws" can make an 85mm lens a beautiful imager on people pics...this is very different than designing a lens that scores high on star charts or USAF targets, or any other "fine-lines-on-paper" type of testing.
Sigma's ART lenses are, to me, a great example of harsh imagers...same with the Tamron VC 24-70 and their new 85 VC...awful pics....great test results on measuring sites.