I'd suggest a freebsd or slack, I can't say enough good things about slack. It's about as close to a Unix as you can get, but still be wholeheartedly linux. (If that ment a whole lot). IMO, If you set up linux from scratch right and the way you want it, it's basicly what Slack was the whole time. (although I wouldn't play down to knowledge you gain from doing that! I tried out Core linux (linux from scratch derivative) and I learned ALOT. Very usefull almost a must.
The one dangerous thing about Slackware is if you get in the habit of installing to many applications from source code in order to get this feature or that feature. You end up chasing down dependances and upgrading for hours.. much more work than it is worth, You learn alot about what prog, or what Library does what, but It gets silly after a while. That's why a whole bunch of Gentoo users came from Slackware. It's along the same lines of OS, but the promise of high performance and cutting edge apps is hard to resist! But then again you end up with a Linux that is fairly better performer then it's i386-binaries based brethren, but reliability drops down to only slightly above that of a newly installed XP. And there is nothing Linux users hate more than having to be forced to reboot as much 2-3 times in a month! GRRR...
I tend to look at slackware as the sort of good backwoods country folk of linux. Everybodies racin' around you trying to be flashy and talkin about how big they going to be trying to be smart and dress in sharp clothes and cars. While you just mosey along, and fish all day long, enjoying the sunlight and good wine. Then when they go to fast and slide their sports car into a ditch, they know you'll always be there to pull them out with your big ol' 4x4. Witness the power of SLack.
Theres a definately good reasoon that SLackware is the oldest distro out there and it is worth a try. hehe.