Best potrait lens for APS-C (nikon)?

What lens would be the most shallow depth of field for out door portrait (where space is not an issue)

sigma 85mm 1.4, sigma 50mm 1.4 or 70-200 f2.8

I already own a nikon 35mm 1.8 and sigma17-50mm 2.8

I'm open to other lens suggestions as long as they give a shallow depth of field.

Also how does the depth of field compare when shooting 85mm 1.4 vs 200mm 2.8 when the composition is the same?

Not all portraits are the same. A 3/4 portrait would work best with the 35mm f1.8.
A quarter portrait with a 50mm f1.8. A tight head portrait will be so much better with an 85mm f1.4. Don't rule out the 70mm end of the 70-200 f2,8 either.

200mm would have you too far back, need a lot of light, and probably make the face look somewhat squashed flat.

First, the biggest mistake made when imaging the human face is working in too close. Things close to the camera reproduce large and things far from the camera reproduce small. When you work in too close, the nose reproduces too large and the ears too small. These distortions of the facial features make a big difference. This distortion might be so minuscule that you and your client can't put your finger on what is wrong. It only takes a tiny alteration to have them denounce their picture. We all have a mental image of what we look like. This is derived from the makeup/shaving mirror. To win a prize you will be wise to duplicate the perspective of this view. Keep in mind that we see an image in the mirror that is twice as far away as the mirror.

The problem has been solved for a long time. All you need do is step back when doing portraiture. Easy to say but not easy to do because when we step back, we create vacant space around the subject so we naturally step forward as we compose. Moderate telephoto to the rescue: If we mount a moderate telephoto, we're forced to step back and this does the trick. So what is a moderate telephoto?

Every camera size (format) has a focal length that is considered "normal". This is one that yields an angle of view of about 45° with the camera held horizontal. This lash-up is a focal length about equal to the corner to corner measure (diagonal measure) of the sensor. For the APS-C, the format measures 16mm height by 24mm length. The diagonal measure of this rectangle is 30mm. Mount a 30mm on the APS-C and you get a "normal" view. OK - what is wide-angle, what is telephoto for this format? Wide-angle is 70% of "normal" or shorter. That's 30 X 0.7 = 20mm or shorter. Telephoto is about 2X of "normal = 50mm or longer. Knowledgeable photo guys choose 2X thru 2.5X of "normal" for portrait work. That's 50mm thru 80mm (figures rounded).
To answer your question: Shorter focal lengths yield a greater span of depth-of-field. Additionally, if the aperture is set to a large diameter when the shot is taken, depth-of-field is shallow. The Sigma 85mm is an ideal portrait lens. The Sigma 17-50 f-2.8 will also do the trick set to max zoom of 50mm and exposed wide-open @ f-2.8. You already own this lens, if you have deep pockets buy more, otherwise learn to use what you have.

I'd go with a 70-200 f/2.8 in a heartbeat. Go to photozone.de to compare sharpness of the various Nikon, Tamron, Tokina and Sigma lens options. The Sigma 50-150mm f/1.8 should be at the top of your list.