GNU/Linux distributions do not have any fundamental difference from each other, so any particular answer would be based on very superficial criteria, which would render it quite pointless.
Moreover, before we make any kind of comparison we must clarify what we are trying to compare, since we can not compare different things and "unix-like" is just too abstract today. Thus we can not compare a Linux distribution or even GNU/Linux as a whole with the Unix standard (which is the only thing that exists today bearing the name "Unix"). We could only compare Unix standard to the Posix standard and we would indeed find that they are very similar. Likewise, we can not compare Debian, which is a current OS with AT&T Unix, which was little more than a reference OS in the sixties, nor it would help if we compared it to FreeBSD, since it is only one of Unix' derived OS and the things that we would be comparing, would defer among the unix derivatives too!
So while there are many things to compare between individual Linux distributions and Unix-derived operating systems, none of them could yield the result that one linux distribution is more "unix-like" than another.
In reality, the term "unix-like" does not bear any significant meaning today, it is only used to emphasize similarities rather than differences.
I don't know how clear this is, but once you work with each OS enough to see the various organizational approaches, in what levels they defer and where they are similar you'll definetelly understand!
Good points made nske. I must have fallen for Slackware's marketing claim to being the most UNIX-like distribution out there. I suppose its position as one of the oldest surviving distros as well as its relatively spartan interface were influencing factors as I first came into contact with the *nix world via HP/UX and Solaris.