My
PCLinuxOS provides the least amount of "culture shock" for a Windows user IMHO.
Mint probably best provides the features most users want with the least amount of hassle.
Ubuntu has the most
momentum, and is probably best for support and day-to-day completeness. There also seems to be more well-written material (i.e. tutorials, how-to's, books, etc) available for it than any other distro..
Debian is great if you really want to
learn Linux and don't mind putting in the time to get there.
Centos, SLED, RHEL, and the
non-Linux BSD family are better if you're planning on pursuing a career in NIX support or development.
And
Sabayon is perfect for those people who haven't the slightest clue about what they're doing - but still want to look incredibly cool while doing it.