My first Linux distribution was Ultrix32 on a Digital Equipment Corp MicroVax II back in 1986 at Lawrence University in WI.
We were building "compilers" for a course.

Mine was Red Hat that I got out of book that friend gave me. I dual booted it with OS/2 Warp. I think it was in 1994 or 1995.

It was RHEL 3, it was actually my friend who gave me with 4 CDs and taught me how to intall and now I teach him how linux works. hahahahahaha

Raspbian!

Ubuntu Dapper in summer 2006. I moved to Mint (Felicia/Gloria/Helena) then tried quite a few distros and now use PCLinuxOS on 2 desktops and a laptop. I've got CentOS 6 and Ubuntu Ocelot in VirtualBox as well.

It would also be interesting to ask what was your second distro. I used Slackware first because that is what came with the book Using linux. My original machine had an unsupported CDrom so I had to keep booting back to DOS to copy the CD contents to disk so I could install the packages. I next used SuSE because it supported my hardware better than the Redhat on the Walnut Creek CD and have been using it since. I also use Centos, Ubuntu and Puppy on other machines.

I'm surprised the suse stable vote is so low. People don't know what they are missing. My 1st version was bought from PC World boxed. The last one bought like that was when Novel took over. Tried one or two others including Ubuntu but have always gone back.

John
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I started back in 2004 with a copy of Mepis and then migrated to Ubuntu 4.10 Warty Warthog actually sent for multiple copies of Warty directly from Ubuntu and gave them out to anyone and everyone I could get interested in trying it.

My first long-term distro was Xandros. Before then I had fooled around with SuSE and Red Hat.

Slackware 1993. I remember downloading sets of floppies. I think the version was point nine something running on my first intel 386 PC purchased from a place called Bizmart.

Fedora Core 1, before switching to Slackware (9?).

Think I tried Mandrake first, to complicated and some hardware would not work. Then I tried Linspire with some success and usability with that.
And then was pretty much like I am now, not very interested in it. Just thought I would try one of the newer distros on and older machine I was retiring. Runs XP great but couldn't get any Linux distro to work, oh well.
Never really sat down and built a machine specifically for Linux distro. I've built about every thing else.
Maybe someday I'll get around2it.
Oh almost forgot I have run XandR os with pretty good success. It seemed to run on a lot more things and was pretty easy to install.

Mandrake 8.1. It was either 'Drake or Suse and I opted for 'Drake as they beat Suse with a new release. Now I prefer Slackware.

Debian

Mandrake(well it was actually a first useful setup) as FreeBSD not in list
a long afer to my great surprise got UbuntuCD by mail... after some time got idea "can i make something production ready usefull on old notebook tuned for my personal habits".. and here we go full linux user