IMO shotgun is the best, if used with hook. Grenade is also good, but it's easy to dodge. Grenade is best in tight spaces where there's nowhere your opponent can't go without taking splash damage.
Except in instagib, I almost never use the laser. It has a low fire-rate, deals only four damage, and fires in a fixed pattern. Though it is instant, it is very easy to predict where the laser will hit you, and therefore dodge. As for the pistol, I only use that when I have no other ammo. It is good for rattling off bullets, but chances are your opponent will kill you if they have a better weapon.
Hammer is nice against people who are pistolling/shotgun-hooking at you. You can kill them up close with the hammer's fire-rate before they can deal too much damage. It's also good against the rifle, since the hammer actually deals damage faster than the rifle. The important thing with the hammer is that you always hook on to your enemy when using it so they can't squirm away and shoot you from a distance.
The katana is nice, but don't count on it. Its attacks are uncontrolled and easy to dodge despite their damage, and it's easy to kill a ninja with grenades. I personally think that the katana is a little underpowered.
I need to figure out something to put here.