What would your favourite class anywhere in the C++ standard be?

Mine's probably stringstream, as I just learned about that and am finding it quite useful.

std::string probably :)

My favorite is std::valarray -- expression templates made easy!

I like std::vector.

std::vector

std::exception (seriously).

To be fair, I rarely use the standard classes.

_Wazzak

If you're like I am, you abuse std::vector to read binary files.
Whereas we should have been provided memory mapped file libraries, a long time ago.

Boost has portable memory mapped file I/Ô, so there is a chance.

shared_ptr