Well, if i follow 'nobody ''ever'' uses' then theres no list, but imo generally the ''least used guns'' are
-XL70E3
-M60
-shotgun
-sawed-off shotgun
-pancor jackhammer
-tommy gun
Thats my guess.
scratch the jackhammer, CAWS and Jackhammer are perfect for Pve, and Jackhammer can be used properly when fighting close quarters (though no one uses it since there are better alternatives) Shotguns are newbie weapons, and are fine.
There is a discussion thread about the tommy gun out there, they aren't going to change it I think.
M60 needs a buff yes, ammo wise.
Hmm, what about the energy pistols?
Cheap starter weapon for EW builds. And pretty deadly to, dont underestimate it. the range is supreme for a pistol, same for damage. But its pretty bad for fast shot people though, but then again, who takes fast shot with EW?
I use it myself to troll with if I want to, or if i need cheap protection. Also it's a great crippler weapon, since it has great range and low ap usage. Heck I could say its the best crippler weapon out there, though you will never damage your enemies if you take finesse along with it.