Best source of protein is food. Stupid looking statement but it is true. I have heard so many 'best' foods for protein including raw egg white which we can't digest. Simple truth, if it has protein and we can digest it it's a good source, if it has more than 17 grams of protein in it and you have it in one go all but 17 grams will not even get into your blood stream unless its just after training then it will be 25 grams instead of 17.
Don't try viewing protein as a miracle food I cover this in the first part of
http://training.fitness.com/articles-research/mythbusting-fitness-files-3-55308.html in detail.
The body needs very little protein for muscle gain and repair and the difference is roughly proportional to the amount of additional energy required to fuel the training. So if you are a couch potato eating 1,500 calories to survive and keep weight stable, the amount of protein you need is roughly 17.5% or calorific intake. You step up and start training meaning you need 2,000 calories a day to fuel the small additional energy use, protein requirement has increased to 17.5% of the 2,000 calories but proportion hasn't shifted. You become a serious athlete eating 5,000 calories a day just to maintain your muscular form and massive training regime, your protein need is now 2.5 times what it was when just starting training, lo and behold still 17.5% of calorie requirement.
Eating just before training has to be considered very carefully. Simple carbs and electrolytes are in and out in of your stomach in moments. Complex carbs can take up to half an hour, protein up to 2 hours, fats up to 4 hours to leave the stomach. There is a small gain to be made getting fuel into your body in the form of carbs and electrolytes before training, but you have to be really pushing it to notice the difference. Protein and fats, which are the major constituents of peanut butter, will not be called on for some time after training, however it could make you feel sick if you are really pushing it.