I would use 'No problem' as an asnwer. But it depends on the context.
I mean, it depends on what you're sorry for, and if the person forgives you.
I think 'Don't mention it' is a little formal.
The most common answers are:
'It's ok' - 'Don't worry about it' - 'You should be, but I forgive you' - 'Thank you for apologizing'
I hope it helps.