The .950 JDJ is the biggest rifle
The biggest hand held gun is the 600 Nitro Express Pistol
One of the largest, based on sheer mass and size is the aptly named "Pig"; or the M60. While not the largest "hand held" gun ever, it is among the largest hand-held, single person (not crew served) weapons that had been in common use. If your looking for length, the Lhati L-39 is the longest single-person weapon in use with a military that has the possibility of being hand-held.
"Best" is extremely subjective and depends on many factors and opinions. A large caliber rifle is a formidable weapon, but it isn't normally what people refer to as a hand-held weapon, even though it's held in your hands. A large caliber handgun is a fairly formidable weapon. A knife can be an extremely formidable weapon under the right circumstances. A knife never jams or runs out of ammo, but a handgun gives you (generally speaking) more range. In order to say what is "best" we would need to know several things. How/where/when will it be used? What is legal where you are? and any other information you can give us about why you need it.
A hand gun.
A handgun.
A sling, or possibly a slingshot.
It is called a pistol
5"
The Battle Axe
you are a nerd. the battlestaff? really? so stupid
The Potato Gun
I can't think of any hand held weapon that isn't manually loaded.
You are probably referring to what is called an Area Denial Weapon.