Lets say you have to go and do an outdoor portrait shoot, but you can only have one lens under $2000. What lens do you take to get the best shots? Assuming full frame
70-200 f/2.8L IS II - this is probably the best choice for flexibility but it won't necessarily produce the very best results
135L - this is an excellent choice, the results will be second to none. The only concern is working distance, IMO.
85LII - unless your client is moving around, the PORTRAIT results of this lens can't be beat by any lens on this list
Sigma Art 85/1.4 - this would be a very good alternative to the 85LII although it's strengths won't, IMO, make it a better choice for PORTRAITS
85/1.8 - not a bad choice but certainly won't yield the best results
70-200 f/4L IS - if you value shallow depth of field, this is a poor choice
50/1.2 - if you shoot wide open and/or via live view and like portraits with more context, this choice is also hard to beat
100/2 - much like the 100/2, it won't yield the very best results.
If I had all of these lenses at my disposal I would choose between the first three options. If the client is moving or there's more than one of them or they want a variety of "looks" (from different focal lengths), I'd go with the zoom. If the clients are stationary and/or there's a possibility of having limited space, the 85LII. Anything else, the 135L.
The 85LII is new to me and I've yet to shoot a portrait session with just this lens however the other 2 lenses I have shot exclusively with them and in the right situation, they're both great choices. The 135L produces superior shots at a similar focal length to the zoom. At this point, the 135L is probably my favorite, again, assuming working distance is okay... like this...