My 85 mm Canon lens has proven to be my favorite over the years for a number of reasons. While not nearly the most versatile, it has always been the most reliable in terms of wow factor, and churning out 'portfolio worthy' images. If you are into portraits, you must own an 85 mm! One awesome factor not mentioned below is that Canon offers a couple different pricing options for it's 85 mm lineup. If you don't want to fork over the big bucks for it's 'L' quality 85 (shown below), they do offer a more affordable option around $300.
1. Proximity - This lens is not too short, and not too long, meaning it doesn't require you to get to close or far away from your subject. Of course this is only important relative to the space you are shooting in (indoors).
2. Lack of Distortion - When using a prime lens like an 85 mm, the proportions of your subject will be nearly exact, unlike when using a wide angle which adds some distortion to your portraits.
3. Bokeh - This is the effect giving you those dreamy blurry backgrounds. Helps you isolate your subject, as well as work in low light.
4. Perspective - You don't have to worry as much about the composition of your shot with the 85 because it keeps the focus on your subject. Unlike a wide angle lens where often times the background is brought into the shot.
5. Versatility - An 85 mm doesn't have to be for just portraits. It can be used in a variety of situations where you don't want/need to get close to your subject.