BeOS ;)

I know, not Linux. But it was my first OS other than the one that came pre-installed.

Can't remember exactly which was my very first Linux distro but probably Knoppix or another early live one.

My first "wipe Windows and install on bare metal" was PC-BSD. I know, again, not Linux.

And again, can't remember exactly the very first "wipe Windows and install on bare metal" Linux, probably Puppy or Ubuntu.