Note: choices about Inventory valuation methods should
not be made without consulting with experts in Accounting and Taxation
practices in your Country. Some countries mandate the use of specific methods in
some scenarios (in the USA, the IRS mandates FIFO if you have any inventory
stored outside the USA) and changing the method used often requires notification
to and/or approval from your tax authority (in the USA, the IRS has specific
procedures and forms to submit). You may or may not have to choose the same
method for all of your Inventory.
Accuracy depends on various the circumstances  and goals of the business, industry practices,
and regulatory requirements. Each method has strengths and weaknesses, and what
might be the most accurate for one business may not be the most accurate for
another business. There isn't a universally "most accurate" method.
 
  | Method             | Best For | Pros | Cons | 
 
  | Standard Price   Manual - a user defined and user managed fixed cost   All units have the same cost | Stable
  product prices Consistent
  manufacturing processes Simple cost
  accounting and budgeting | Good for monitoring
  material / labor / overhead variances | Differences between
  actual cost and standard cost could over or understand inventory value May not
  reflect market prices   | 
 
  | Average Cost   Automatic - calculates a weighted average  
 
  All units have the same cost | Relatively stable
  product prices High-volume,
  homogeneous products Very common | Smooths out
  fluctuations in purchase prices   | Does not
  track specific unit costs May not
  reflect market prices   | 
 
  | First in First Out (FIFO)   Automatic – calculates oldest to newest costs | Perishable products Prices that
  are increasing   | More accurate
  during inflationary periods Accurately
  measures the cost of replacing inventory | May lead to
  higher taxable income during inflationary periods * | 
 
  | By Lot/Serial number   Automatic – calculates the real costs   Also called specific identification | Regulated industries High value products   | Precise costs   | Requires lots
  / serials to be entered on all Transfers | 
 
* since you are selling your older, cheaper inventory your cogs
will be lower and your profits higher than if you had used another method such as
average. 
 
See also  https://www.odoo.com/documentation/master/applications/inventory_and_mrp/inventory/product_management/inventory_valuation/inventory_valuation_config.html