What do you think is the most frequently used greeting in Japan? In my opinion, it is otsukaresama（お疲れさま）.

The word and its derivations are very useful but you need to know how to use them properly. Or there is a chance of getting regarded as being rude.
 
In principle, otsukaresama ans its derivations are used in work-related situations. They are typical greetings before and after worktime. In a close relationship, these words are exceptionally used as normal greetings.
 
parting couple
 
otsukaree（お疲れー）
・(very, probably too) Casual
 
・Can be used both as hello and as good-bye.
 
・Don’t say otsukaree to your boss, customer, nor someone you have just met. If you do, they will consider you as a rude and superficial person.
 
 
otsukaresama（お疲れさま）
・Casual and polite
 
・Can be used both as hello and as good-bye
 
・Don’t say otsukaresama to your boss, customer, nor someone you have just met. The word sounds more polite than otsukaree, but not enough for these people. You need to put “desu” after otsukaresama.(see below)
 
 
otsukaresama desu（お疲れさまです）
・Polite
 
・Can be used both as hello and as good-bye
 
・This one is polite enough to use to your boss, customer, or someone you have just met.
 
・Since it is a polite word, I rarely greet with the word to my family members or friends. Using it to them is OK, but it may sounds distant.
 
 
otsukaresama deshita（お疲れさまでした）
・Polite
 
・This one is also poite enough as otsukaresama desu.
 
・It is a past tense. Therefore it can be used as good-bye, but NOT as hello.