I believe if someone says, 'Much obliged,' (which seems to contain more gratitude than 'Thank you') the response both logical and polite is, 'Not at all,' since the first person is saying, 'I am indebted for the kindness and thus owe you a favor,' while the second replies, 'Please, no, you don't owe me anything.' But if the first person says, 'Thank you,' for the second to answer, 'Not at all,' is illogical: the first says, 'I am grateful to you,' so such an answer would imply, 'No, sorry, you are not grateful,' which of course is not what is meant, or understood. Still, I believe it is understood in the idiomatic way, and thus it is still polite. 'You're welcome,' means, 'I did it for you gladly and will gladly do it again.' 'No problem,' though doesn't have such connotations of generosity, implying only that the action depends on its being particularly easy, something one might do only because he didn't have to go at all out of his way or to any expense. And of course, 'My pleasure,' is suitable in any case. Conclusion: Let's start saying, 'Much obliged,' more often, i.e. any time at least a little more than, 'Thank you,' seems necessary.