Whoa, whoa, whoa! I think we are all forgetting some of the worst weapons in video games. Yes, people have mentioned some bad ones (Pistols in Fallout 3, Any Halo weapon, Farcry 2 weapons etc, etc) but here are a few that people have seemed to neglect.
Killzone 2 - The magnum pistol that took longer than the normal pistol to fire, held 6 bullets and you needed to hit people about 8 times to kill them. Unless you shot them in the head, in which it only takes 7.
Gears Of War 1 - Magnum. Why? Seriously. The standard pistol you start with is once again better, nevermind the Lancer or any other weapon. If you were down to the magnum you were better off trying to meele the opposition to death with your shotgun.
Final Fantasy Tactics - Daggers, Flails, Axes...pretty much anything other than swords, guns and...well that's pretty much it. Oh, and level 5 death in Final Fantasy 7.
Oblivion - The meele combat system. Oh, wait...Sorry, that's merely a game complaint. Lemme rephrase that.
Oblivion - Any meele weapon. (Far better)
Dawn Of War 2 - The Flamer. Useless, useless weapon. You are better off using the bolter, and even that is...well, quite shocking.
Day Of Defeat - Any automatic weapon. The recoil is much like having sex with an AIDS infected prostitue who is the size of an elephant during mating season. Though it is quite amazing when you actually kill someone with a machine gun in that game. It's honestly an achievement.
Counterstrike - Any Pistol but the USP, the glock and the Deagle? Let's not forget the UMP and all those useless SMGs apart from the MP5.
Final Fantasy 7 - Cath Sith's limit breaks. Yes, all of them.
You could go on and on with this, but really it's quite shocking how bad some weapons are in games.