Portrait lens... Something around 135mm in effective focal length that can shoot fast and deliver smooth background blur.
On an FX body I'd reach for my 70-200mm f/2.8 and for a DX body something like a 105mmn or an 85mm would be what I'd want. Fast glass (f/1.4, f/1.8 etc) would be my first choice so I'd have a wide latitude when it comes to depth of field. .....