To me, one of the greatest weapons to ever have in a video game is MegaMan's Metalblade from MegaMan 2. I think it's one of the most memorable weapons for a lot of people. Anyone that has played that game knows the joy of switching over to that weapon and using it for basically every situation. And if my memory serves me correctly, it's one of the few weapons that will easily kill the boss it's taken from.
Surrender to all Life Beyond Form
Lance of Longinus on Bandcamp
Noise Redemption Prophecy on Bandcamp