천만에요. cheonmaneyo. 아니예요. aniyeyo. (It's not a big deal.) 뭘요. mwolyo. (I didn't do anything.) -> 뭘요? means "for what?" so not to say in question.

Well, I don't like to jump in. However, 천만에요 is what we see in the foreign movie with Korean subtitle, not what we say it in real life. I'd like you to say 뭘요 in return for "Thank you.", I think it's the most polite answer.