@ Xeras'na Bladewing
In singleplayer the Widow is actually more powerful damage wise, thought the Jav is arguably cooler in its effect. In Multi the jav does more damage, but only slightly.
I don't know about best weapon, but my new favorite is the Ullapool Caber in TF2.
I've made it so that my demoman now has no other attack, instead charging at people with a live grenade his hand and smacking them with it. It 1-hits everything but the heavy and people who are overhealed, and when it crits it can kill multiple targets.
Somehow it's actually good enough to be a valid tactic.
Sometimes the craziest tactic is the best. Utterly stupid in theory, extremely effective in practice. I've had good success going full blown melee fiend, especially if there's a heavily-grouped close-quarters firefight. Better yet if the game allows, you're a one-man third team going melee fiend in said close quarters firefight. :3
My absolute favorite would have to be the Hand Cannon from Resident Evil 4.
You play though this dark and atmospherically creepy game, and you get your butt kicked a couple of times. Then on the second play through, you can buy a .50 caliber revolver which you can upgrade to have infinite ammo with no reloads. It's pretty much just a step below having a rocket launcher. You can end any boss battle in two shots!
It turns a classic Survival/horror game into a HILARIOUS shooting gallery.
Flak Cannon from Unreal Tournament 2004. The best version of the gun, to me. The UT99 version was stronger, but the firing arc on the alt fire grenade was a little too low, and the UT3 version's spread was comically large.