this type of question always seems to lead to the same thing... someone says that gun X is terrible and then there are a ton of people who jump in and say "no, you just don't know how to use it, it's the best gun in the game"...
There is no worse gun. If you beast with it someone else might not. It depends on the person.
I haven't used all the primary weapons but the worst I took to gold is the Vector. That weapon took me 2,512 kills to get gold and I never touched it since then. That weapon punished my stats and I averaged 1.5 KD using that weapon. I find it unreliable and unsuitable for my r&g playstyle.
Honestly... This is situation based. You can't just go out and say THIS gun is the WORST. There isn't a worst gun in this game, there is only a worst gun during a certain situation. Take Hydro.. as an example I would say something that could be considered a WORST gun would be any of the shotguns. I am not saying that is the case I am saying it as an example.. Say Hydro, for Team Deathmatch. Very long lines of site and Team Death match does not give you a reason to be out of place.. like running for a cap point, or a tag.. So you are up against people hiding behind cover with assault riffles and sniper rifles. Yes, you do have runners with SMGs but they still got you on range, and you have very limited close quarters areas in Hydro... BUT.. the game is over the next map is Hijacked, Domination. Now you have a very good advantage with the shotgun and the snipers are hurting unless they really know what they are doing. On hijacked domination any of the shot guns can rock someones world.
So it just depends on the map and game type.. plus how good you are personally. I mean as far as worst weapons go. I personally would pick a sniper rifle. Alwaus causes me to lose gun game because I can't hit crap with them.
It's a personal worst, mate. It's not a generalization of the weapon. It can be beastly on the right hands and/or gametype but it's just not a weapon I'll pick if I have a choice.