Gonna go with a few.
Not really a weapon, but tanks. As Yahtzee says "Spam tanks and steamroll the map and hope the objective is something that dies along the way" -Only in FPS, you have your team do this, and hope the enemy doesn't counter spam Anti tank rockets... This works surprisingly well in some FPS, much like in some RTS...
For actual guns... - SNARK! I think from HL1. Had so much fun with those little buggers. (Naturally, Snark pit was one of my favorite maps)
Grenade suicide runs. BFG, supercharged Grav Gun from Halflife 2, Grenade launchers, Sheva's Longbow from RE4. Engineer wrench from TF2. And thats all I can remember...