My all time favorite lens is the 50mm lens. I learned almost everything I know about photography with the classic 50 focal length. I've love to sell and trade lenses to try different things but the 50mm lens has remained a constant in my bag. A 35mm lens has a look that is similar to what the eye sees, however it's not as interesting as the 50. The 50mm is pretty similar but compresses the background a little more and has less distortion.
When photographing portraits it's my go to. It has nice compression but doesn't completely compress the background so your subject still has some of the environment in it. This is all really personal preference but in my opinion the 50 is the best for portraits. If I had to sell all but one lens it would be a 50.
I couldn't imagine my camera kit without a 50mm prime. I photograph almost everything with either a 50mm or the equivalent. In my opinion it's the lens you can't afford not to have. When it comes to portraits its perfect. However, if you're primarily a headshot kind of person you may want to pick up a 70-200 or 85mm as it will have less distortion. The 50mm doesn't do the greatest for close up portraits. Below are some of my favorite images taken with the 50mm