Ai-S 50mm F1.2
It gives a shallow DOP which helps greatly with keeping the attention on the subject, and produces "approachable" image quality, meaning it's not razor-sharp like, yet not soft in a bad way. I also love the quality of colors from it. It's also my most used lens given that I do mostly travel/journalistic kind of shots.
Using telephoto lens for portraits gives a feeling that you are seeing the person from a distance away (duh). For portraits, I don't like that, unless I want to portray an "observer" kind of perspective.