I agree with Whickerman, but here in Australia everyone seems to say, "I'm good." It grates with me - when I enquire after their health I don't want to know how well-behaved they are!
in Britain, fine or well. There's no particular logic or grammar to it - 'How are you?' is basically an inquiry after your general health and wellbeing, and you should be able to say your health is 'good'. But Brits usually don't. Good is not uncommon in other English-speaking countries, but I think they also say 'well'.