All linuxes are very close if properly maintained, but I find Debian is still the best for security and package management. Linux is not the best for performance, no matter what flavour you have.

That said, FreeBSD has a superior kernel/IO performance but is a bit messy to work with.

This project should really improve things when its done, GNU Debian on top of the FreeBSD kernel!!
http://www.debian.org/ports/kfreebsd-gnu/