My original one, Darkie... I'm pretty sure I had literally no motivation for that one, I think I just thought it sounded cool back when I was new to TWHL/the internet.
Scotch comes from the fact that that actually is a nickname of mine. I did realise this conflicted a bit with Archie, but I'd been looking for a change from Darkie for quite a while. Nowadays any of my usernames are derived from Scotch, usually "Scotch22" (which I thought sounds like Catch-22) and "ScotchArtist" (no idea why, I'm not an artist).
My Steam username is nearly always Scotch, but recently I've had Zap Rowsdower (a major character from The Final Sacrifice, which I only know through Mystery Science Theatre 3000) and Skötchtoberfest (just because).