What lens depends on what you want to accomplish.
For a studio type shot, you can get a rough feel for what you want by going back and forth with the calculators here: http://www.tawbaware.com/maxlyons/calc.htm
What you'd do is take the space you have, and the amount you want to have in frame, and use that to figure out how far from the background you're going to be able to get your model. That lets you figure out how much depth of field you can work with for a focal range.
Using that all together tells you what focal length and aperture you need for a given type of shot.
Here's an example, because that probably sounded like gibberish.
I've got a room that's 12 feet by 12 feet. I want to stand at least five feet from the model. So it'd be:
Backdrop ------ (??? feet) -------- Model ------- (5 feet) ------- Camera
I use the dimensional field of view calculator, and determine that a 75mm lens will give me a headshot (1'6" of view) that has from about and inch and a half in focus at f/2.8 to four inches at f/8. Furthermore, if I have the model posed about 5 feet in front of the backdrop, I can change the distance to subject in the dimensional calc to 10 feet to know that if I stand with my back against the wall, the backdrop needs to be at least 3 feet wide to not have the edges show up in the frame. For a head shot, that's less important.
However, let's take a full body shot:
Backdrop ------ (5 feet) -------- Model ------- (5 feet) ------- Camera
We'll say my model is about six foot tall, and requires roughly a foot on each side to get a comfortable full body shot without crowding the frame. I'll use the calculator with trial and error to get a field of view of 8 feetish. Holy crap, suddenly from five feet away, I need a 14mm lens! Not only that, but I need to make sure I have a 120 inch wide backdrop and pretty tall ceilings.
So maybe my distance of 5 feet isn't reasonable, since I can't back up any more, I'll have to push the model closer to the backdrop. A decent 24mm or 28mm is going to be much more flattering than a 14mm (even with barrel distortion corrected), so I'm going to see what distance I have to put the model at using one of those. I find I can just barely get away with a 24mm if I've got about 8 feet to the model. Now the backdrop is only going to be 2 feet behind them, so I'll want to crank up the aperture to decrease the focal length. At f/8 it's ginormous! Everything in the room is going to be reasonably sharp. If I can crank it down to 2.8 AND FRONT FOCUS, I can slide the backdrop to the far side of the focal range, just barely.
I know that's me rambling about something that may not directly apply, but I hope it gives you a sense for the kind of tradeoffs you need to be considering, and why a one size fits all answer is meaningless. Any good lens can be used, if you've got a grip on the relationships between distance and space. If you get a strong grasp on when wide angle lenses can be flattering, and when telephoto lenses are flattering, and how much space you have to work with, lighting, etc etc, which lens to use falls out of what kind of picture you want to take.
But Mom, Pentax IS rebellious
Pentax K-7, K20D
Pentax SMCP-FA 35mm f/2.0 AL -- Pentax SMC 50mm f/1.7 -- Pentax DA 50-200mm f/4-5.6 ED -- Sigma 28-70mm f/2.8 EX DG IF Aspherical -- Pentax DA 18-55mm f/3.5-5.6 WR