cURL makes a nice comparison: http://curl.haxx.se/docs/ssl-compared.html

GnuTLS had its own problems recently (see: http://www.gnutls.org/security.html), and regarding NSS... I couldn't find a proper public "security" page for it (eg, release notes for 3.16 point to CVE-2014-1492; but the link to the bugzilla issue is not public), so I don't know.

Obviously a good security record doesn't imply things like a good code base, good practices, etc; (although it could).