what is the "standard" Signatures library on CPAN today?
There is none, all of them have their benefits and drawbacks. For the record, the Moose community has no preference. And MooseX::Method::Signatures is part of MooseX::Declare which is still considered an "experiment" by many (including much of the Moose core dev team).
So I'm trying out Moo, which I saw used on CPAN.
Do not let a few brave souls lead you to believe that Moo is a not still highly experimental. I would be very wary of deploying something so young as Moo, I would lead more towards the pure-perl version of Mouse which is much more mature.