To build muscle successfully requires an enormous amount of discipline, effort and attention to detail, with attention to detail having to cover a number of different aspects including training, diet and nutritional supplementation.  Too much attention to one aspect at the expense of the others unfortunately generally results in less than satisfactory muscle building gains.

To optimally build muscle we need to strike a balance between intense training to stimulate muscle growth, sufficient rest to allow for recovery, balanced nutrition to provide the nutrients required for growth and intelligent supplementation to augment all of the aforementioned.
When it comes to nutritional supplementation, the vast array of confusing and often conflicting information coupled with the huge amount of marketing thrown at consumers is a double edged sword in that it often leads consumers to either steer away from nutritional supplements completely or on the other hand to overemphasise their use.

As a competitive athlete myself, I’m here to tell you that the reality of the matter is that there is no way a hard training athlete will ever physically perform at their best and recover & build muscle optimally without nutritional supplementation.  Even the most well planned and thought out eating plan, based on whole foods alone, will battle to deliver the increased protein (amino acid), essential fatty acid and micronutrient requirements of someone undergoing intense physical exercise on a daily basis.  From first hand experience I can say that without a shadow of a doubt I would not have been able to build the level of muscularture I carry today without consistent and intelligent use of nutritional supplements.
From personal experience here follows my pick of the 10 most valuable muscle building nutritional supplements:

Whey Protein or Whey/Casein Blend
Protein provides the essential amino acids required to repair muscle damage after training as well as build new muscle tissue.  I recommend aiming for 2,5g to 3g of protein per kg of bodyweight per day.
Milk proteins (whey & casein) have an extremely high biological value and are rich in essential amino acids, particularly the all important branched chain amino acids.  Whereas whey protein is rapidly absorbed making it the best post workout protein option, casein is digested and absorbed more slowly, providing a steady stream of amino acids over a longer period of time thus making it an ideal protein source between meals and before bed.
My recommendation – take a serving of a whey protein  containing shake immediately after training (ideally supplying 30-50g of protein & 50-100g of carbs) and a serving of a casein or casein/whey blend before bed (ideally supplying 25-40g of protein).

Creatine
Creatine supplementation boosts intra muscular levels of creatine phosphate.  This in turn yields two important muscle building benefits: 1- Increased strength, lifting performance and recovery as a result of optimised ATP production & 2 – Enhanced cellular hydration thus providing a muscle-cell volumising effect, which adds to immediate size as well as stimulates pathways that turn on protein synthesis leading to long term muscle gains.
My recommendation – take a 3-5g serving of creatine with breakfast (or other suitable high carb meal) and an additional 3-5g on training days with your post workout shake.