The combination of the 85mm 1.8 and the 135 2.0 is very hard to beat for people photography. I also have the 100mm F/2.0 and it doesn't get used much any more. I made an effort to find a job for it recently and found one... taking pix of classic hood ornaments at an antique car show!
The Sigma 85 F/1.4 is interesting. I've been reading consumer reviews and there seems to be a high percentage of bad lens reports out there. You would definitely want to buy it from a place with a liberal return policy and be prepared to test it extensively on receipt.
Here is a shot with the 85 1.8 at F/2.0 on my 5D3
Here is a shot with the 100 2.0 at F/2.8
And here is one with the 135L at F/2.8
Basically, any of the lenses will allow you to easily separate your subject from the background with shallow DOF. It just depends on how big your subject is and how far away you want stand.