Most accurate method of determination of working length in a tooth with open apex:
Moisture on paperpoint
Electronic apex locator
Other digital method
Both 1 and 2
Advantages of apex locators

Provide objective information with high degree of accuracy.
Accurate in reading (90-98% accuracy).
Some apex locators are also available in combination with pulp tester, so can be used to test pulp vitality.

Disadvantages of apex locators

Can provide inaccurate readings in following cases:
	
Presence of pulp tissue in canal
Too wet or too dry canal
Use of narrow file
Blockage of canal
Incomplete circuit
Low battery


Chances of overestimation.
May pose problem in teeth with immature apex.
Incorrect readings in teeth with periapical radiolucencies, and necrotic pulp associated with root resorption, etc., because of lack of viable periodontal ligament.