'Hi' works best. People vastly overstate the actual need to use To-whom-it-may-concern's and Dear-Sir-or-Madam's. I tend to find it's not something anyone actually ever uses in real life, it's just a formality taught at schools when 'formal' language is considered. Of course, this is not 100% true, but you can't go wrong with:
Hi,
I am writing in relation to...
Blah blah blah.
Kind regards,
Me.