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.