Ubuntu, Mint, Debian, Opensuse and Fedora tend to be quite popular choices of distro, but I don't think there is any such thing as 'one true distro'.
Peoples views are different about Gnu/Linux and it's many different flavours.
What one person prefers, another may detest!
The other thing to remember is that all Gnu/Linux distros are a combination of the Linux Kernel bundled together with the complete Gnu toolchain plus lots of free software and the odd bit of non-free proprietary software here and there (the inclusion of which tends to be highly controversial in some circles!)
So if anything, as Linux IS the Kernel itself, I guess you could say that the Linux Kernel is the one true, pure Linux!
Also, there are efforts being made to standardise Gnu/Linux, try googling 'linux standard base'.
The aim of the LSB project is to standardise the entire system structure based on 'POSIX', the 'single UNIX specification' and several other standards to ensure maximum compatibility (including backwards compatibility) between Linux distros.
So in a sense, as long as all distros follow the standards set by the LSB, there would be no need for 'one true' Linux distro, as they would all be standardised.
Those are my thoughts!