The best plumb-bobs are perfectly balanced and symmetrical. A perfectly balanced plumb-bob should produce accurate readings, especially when transferring points from level to level.
It can be difficult to tell just by looking or handling the tool, whether or not it is perfectly balanced, but as a general rule of thumb – if your plumb-bob appears to be a symmetrical shape then it will likely be balanced.
String line
The best string line to purchase is a nylon or polyethylene braided type. These string lines are will not easily stretch, they are water-proof, rot-proof and resistant to mildew.
Braided lines also have the added advantage of not spinning (or at least not as much as twisted string lines tend to). This is particularly useful as the tool must be steady in order to make take readings and transfer points accurately.
Screw cap
Purchasing a plumb-bob with a screw on cap is advisable as this ensures that the string line is perfectly centred. Having a string line that protrudes from the dead centre of your plumb-bob will help to ensure accuracy and balance.
Fine point
Plumb-bobs with a ultra-fine point are advantageous as this makes pin-point precision when marking and transferring points much easier.