Ubuntu seems to be becoming 'corporatized/commercial'.
Fedora was okay, but I never was able to configure some desktop settings.
PCLOS was the best/fastest, except for Broadcom wireless (I have 2 B/C machines).
SuSE was FAR worse than Ubuntu - way bogged down.
CentOS was my first Linux OS, I knew nothing about CLI then (still don't, but I can figure it out now).

Anybody with experience on LOTS of versions? What are the 'leanest' versions?

I don't 'game', or do anything that's resource-hungry. 3 of my 4 machines are pretty high-end new hardware.

Yeah, but first tell me briefly about your machine -- CPU, RAM, etc. Okay, I see you say "modern high end" but what about the slowest?

Look at the first two of my distros used.

"Tiny Core Linux is a very small (10 MB) minimal Linux GUI Desktop"
http://tinycorelinux.com/

http://www.slitaz.org/en/ only 30mb

next would probably be Puppy then DSL.

Arch, Slackware and Debian can be as light as you want or as heavy depending on what de/wm you install.

Yikes - looks like a few of them are just a few lines of code above DOS!!! Maybe 'pretty thin'??? Perhaps a distro with support in this sub-forum?

The oldest machine has XP on it; I just wiped Ubuntu. It's Dell D600 Pentium M 1.6G / 1G. Still pretty quick for 1.6, but not as quick as the other 3, all 'multi' core AMD's.

Thanks............

Dell D600 Pentium M 1.6G / 1G is fine. The problem with ubuntu is all the services that start by default.

Beats my old desktop that has a tiny Via 1.4, 2 gb ram and a pci nvidia 5200. Or my kids OLPC Xo, with a 400mgz cpu and 256mb ram, it is running Debian with e17 only thing slow is the actual boot up and that is because it is running on a 8gb sd card.

If it was me I would go with Debian with xfce or lxde, or Arch.

That depends on what you mean by thin. As far as distros with forums here, Slackware runs lean, but a full install is anything but lean when it comes to disk space. Arch starts thin to the point of being a shell, a network connection and a package manager at the end of the initial install as far as I'm concerned. You'll have to add X and a DE yourself.

If you don't want a large corporate style distro, try any of the smaller distros, Mint, Mepis (my favorite) AntiX etc are built by smaller communities. Look at distrowatch and stay out of the top 10.