All 3 languages (Its actually UnityScript, not Javascript btw, they are related but not identical) have "full"(There are a few restrictions in the free version of Unity but they apply equally to all 3 languages) access to the .Net framework(or mono rather), they all get compiled to MSIL and can all do exactly the same things, Use whichever language you like best.

C# is probably the most popular one as it also has a solid community outside of Unity, UnityScript is only used by Unity and Boo is kinda obscure (It is quite different from Python)

I would suggest C#. I have used UnityScript primarily before, but I switched to C# a while back and love it. You can get Microsoft Visual C# Express for free as your C# code editing software. It has a lot more features than the editor that comes with Unity (UniSciTE).

C# has some features that aren't available in UnityScript, and I think the language is overall just more robust and powerful. A lot of more advanced users in Unity will be writing their code in C# as well (at least that's what I've noticed), so it'll help you later to be able to read their code efficiently when you're looking for help.

On a related matter, I'm going to have to pick up Unity soon, and will probably be doing that with C#, which I haven't used before.

I have a day job writing C++, and have dabbled in a lot of other languages, mostly functional. I'd like to find the closest equivalents of Stroustrup, Meyers, Sutter, Stepanov and Alexandrescu in the C# world. Any authors, books, other sources you guys would recommend?

There isn't the equivalent of Effective C++ or another must own C# title. C# Programming Language is probably the closest equivalent, and Anders Hejlsberg ( language creator ) is one of the authors. It's a good book, but I havent owned a copy since the first or second addition... it's up to the 4th edition right now, so it's technically one iteration behind. The big addition to C# in the 5th revision is async programming, and frankly It doesn't look all that hard to grok.

In your situation, with prior language experiences, the In a Nutshell series is pretty aptly named. The prior recommendation though is probably "the book".

I'm still a big fan of UnityScript, and it has a fair amount of traction among Unity developers. But if you go the UnityScript route, you'll end up needing to know a fair amount about C# anyway - all the class libraries you'll be using are primarily intended for C#.

I'd recommend staying away from Boo. It is an interesting Python adaptation from an academic standpoint, but has very little real-world support, and a surprising number of common Unity tasks are very non-intuitive.