First of all, it's a matter of taste. All 3 languages are fully capable. It's also worth mentioning that performance-wise there is no significant difference.
So, which one? You'll have to choose for yourself.
Perhaps this would help:
C#:
- pros:
- well known, easy to find people who know it
- strict (find mistakes during compile time)
- usable code bits may be found online, even outside Unity's context
- cons:
UnityScript (not really JavaScript):
- pros:
- cons:
- not strict (find mistakes late - only during run-time)
Boo:
- pros:
- beautiful syntax (pythonesquely clean)
- strict (find mistakes during compile time)
- cons:
- not many people use it (so if you're hiring...)