hmmm
either my Light Machine-gun,
or the Assault Carbine.
I used the latter more often as it was accurate and dependable.
while the Light Machine Gun, could totally destroy any enemy I pointed it at.
both weapons were obtained when I visited Black Mountain first time because I thought the friendly mutants were nearby, nope just some Dumb dumb's and and Nightkin. most of the super mutants had assault carbines and i was pretty certain it was a good gun, so I took it, it was by my side ever since.
The light machine gun came later. I figured the Mutants should have re-spawned with better loot at the time so i fast traveled.
a party of Nightkin armed with my Future Death sprayer was their to greet me. I decided the Light machine gun was superior to the Marksman Carbine, so i never got it.
also I got the C-Finder for 20 caps but never used it.
except once vs the Legate, but I don't think it counts as I had to reload because the legate Had survived
THE CONCENTRATED POWER OF THE SUN.
and killed me.