This is important because Soldier already has the Vindicator/Mattock, allowing you to pair two amazing assault rifles with an amazing sniper rifle. The M-98 Widow is obviously amazing for Infiltrators, but Soldier gets to pick this weapon up too. High damage, lots of headshots, lots of dead bodies.
Despite being the most badass weapon in this category there isn't much to talk about that hasn't already been said. It's basically the Mantis but with higher penetration and damage, allowing it to one-shot most enemies even on Insanity. Have you ever wanted a portable railgun strapped to your back? Well, the M-98 Widow exists for that sole purpose.
The Widow has a massive cult following for a good reason.