There is no ideal 'portrait' lens. There used to be a notion that focal legnths of around 80mm-135mm gave the most flattering images, but portraiture has moved on so far since then. I've just won a comp taken with a 20mm lens, I also use my 200mm for portraiture.
Fast lens are often used for portraiture because of the ability to keep the depth of field on the subject and blur any distracting background. A 50 prime with either 1.2, 1.4, or 1.8 max aperture will allow you to focus on one eye and have the other soft if that's the look you want but the fixed focal legnth can be restrictive. One lens I would recommend is the Sigma 50mm-150mm f:2.8. I've had mine coming up to a month now and it's a great lens for portraiture and only cost me a little over £300. That's not much good if you're wanting a full length lens though so you'd be best looking at the f:2.8 wide/standard zooms for that.