I would say Back|Track is probably the best well-rounded distro available. I've used Deft 5.0 and it is mainly a forensics oriented distro, I've also used Nubuntu which is a decent distro, lacking in tools though in my opinion. I've also used STD but don't think they've had any updates in a while (I aplologize to their team if I am mistaken) and I've used Pentoo which is close to BackTrack but on a Gentoo base. And then there is also Matriux, a somewhat new distro with pentest/forensic tools as well.
You can add most of the extra forensics tools to Back|Track with very little problem thereby giving you, imho the best, most well-rounded distro.