rarity is basically based off of how many active users a country has.  if a country only has 3 users then that would be really rare that you would be able to get that flag.  if you got a flag with italy with over 1,000 active users then that would not be very impressive.
That's basically what I'm doing.
Bang out the rare flags now and then cruise in on the big countries.
9 of my 31 flags are from nations with 25 or less users, 6 of those from nations of 10 or less
 
                                
                                
                                
                                
                                
                                
                                    NO ONE at this table ordered a rum & Coke 
 Charles: Penn has some good people 
 A CT? Really? 
 Any two will do 
 Any three for me 
 Any four will score 
 Any five are live