"Best of luck!" and "Good luck to you!" are very similar in form. The one that encourages your friend more would depend on how formal you wish to be or the amount of "luck" you wish to give, in essence. "Good luck" is a less formal and simpler way to give someone 'good wishes' and well being; whereas, "Best of luck!" is more intense and should give a stronger connection of wishing your friend the highest amount of luck you can give him or her. What it really comes down to is the sincerity. Similar to saying "I love you" to someone as an example. The difference is being whether you write it with more expression or not like, "I love you!" or just, "I love you." There's "Good". There's "Better". Then there's "Best!".

Best of luck and good luck are basically the same. I often say something like "you can do it" or "don't worry, you'll boss it!" (which just means you'll do a really good job of it. Other ways of wishing somebody good luck include: Break a leg (what you say to actors before they go on stage or somebody about to act in a play etc) Knock em dead / blow them away - used when you are trying to impress people (in a good way) in like a job interview or something. Laurence

I think you've hit upon it--"best of luck" is just a more intense form of "good luck".