What is the most common and profitable investment for a good retirement in Australia?
In Australia anyone thinking about retirement should be concentrating on superannuation. Contribution is compulsory (I think the current minimum contribution rate is 9.5% of salary) and both contributions and investment returns are very tax efficient. The Government site is quite comprehensive - http://www.australia.gov.au/topics/economy-money-and-tax/superannuation - have a read and come back with any specific questions.