What mm prime lens for a Nikon would be good for portraits in low light situations?

I'm thinking either 24, 55 or 85mm of course a fast lens. I would like to use it in weddings so basically as an all-purpose until I can start looking at fast zooms. (Back of shooter).

Most portrait photographers would recommend an 85mm lens on a full frame body.
So, if you've got a full frame body, then go 85mm.
If you've got a crop, then 55mm would probably be a better choice. (55x1.5 = 82.5mm equivalent)

I use up to a 110mm. Low light conditions are the critical variable here. Get the fastest lens you can find. With portraiture, distance isn't a problem, so don't worry about focal length. You just stage the shot differently.

24mm? Too wide, if you view the images at length you'll notice the perspective compression

50mm or 55mm. Gives a natural Field of View on a crop sensor camera. There's still a little perspective compression, but not enough that 99.9% of people would notice it.

85mm? On a full frame camera this is the ideal. The Field of View is natural and perspective is "normal".

Someone suggested a 110mm lens? Most people would find they have to stand too far back, and framing could start to be an issue.

For portraits, the 50mm f/1.8 would be the best for its price. If you can afford the f/1.4 version that would be better. For weddings, the 18-55mm is good enough if you know exactly what you're doing.