For commissions, I only use the highest quality lightfast colours which ensure that the images will last and not fade, but unless you are selling your work, do not get too hung up on this. All my pencils can be bought separately which means I can always buy replacements.  

I would encourage anyone starting out in portraiture to invest in a few individual better-quality pencils in the colours you need, rather than buying sets containing colours that you will not use.  

Saying that a new set of pencils is a very fine thing indeed.

If you are drawing everything and anything, then go for a set.  Needless to say, the larger the set the wider the variety, but do check the pricing as they do not always work out cheaper than buying the pencils individually. It's worthwhile checking sites like eBay as you can often find lightly used sets which are being sold on.

Faber-Castell Polychromos are my workhorse pencil. They are a good all-rounder medium hardness pencil providing a solid base for layering on the papers I use. The Warm Grey 2 is my go-to when drawing outlines, and to use as a smoothing layer between colours, as I tend not to use blenders that often.

Caran D’Ache Pablo are a much softer pencil, which I use for adding the shading and depth in my portraits.  I particularly value the Granite Rose and Cocoa when drawing tongues.

Caran D’Ache Luminance are probably the best quality pencils on the market, they are also one of the priciest.  The range of colours are comprehensive, with their percentage colours such as Warm Earth 5%, Butternut 10% and Sepia 50%, particularly useful for highlight and shade.

Derwent Lightfast are top of the Derwent range and have some useful colours such as Mars Violet and Mars Black.

Derwent Drawing are the softest pencils I use.  They have a thicker core than my other pencils and a chalky texture.  On Grafix I use them on the reverse side of the paper, to get real depth and intensity to the portrait. I would not be without Chinese White or Wheat.

Derwent Coloursoft (7-8). A set of 24 were the first artist-grade colour pencils I got, years before I started drawing portraits, and was mindfully colouring in mandalas and the like. They are lovely pencils and blend well, but sadly for me not all colours in the range are lightfast, so I only use those rated lightfast 7 or 8 now.

Holbein Artists Colored Pencils (***). Like the Derwent Coloursoft, not all in this range are lightfast, so I only use a limited number which are labelled ***. Their sand and creamy/ivory colours are very good for using as a later layer on golden coloured dogs.