My advise is JavaScript, for the following reasons:
First, out of the three languages provided (C#, JavaScript, Boo), both C# and JavaScript are well-known and used in industries (I have never even heard of Boo prior to Unity). As a result, those two scripts will be useful if you are to ask for programming help, or will one day go into programming in the industry.
Second, of the two mentioned above, JavaScript is very lightweight and simplistic to programming. What I mean is that while in C# you have to write a class declaration and similar, JS automatically takes care of that, reducing the amount of busy work you have to do.
Third, which may be the largest factor, many, if not most, of the Unity scripting tutorials I have seen are done in JS.