In a business environment you most often use "best regards" or just simply "best". "Sincerely" is most often more personal used with friends and family. Hope that helps!

Those are both good options, and you often don't even need a formal closing in an email to someone at your own company. Please note that in your question, the correct case would be "who," not "whom." You could also omit the relative pronoun and the verb to be: "… someone working at your company."

In my opinion, the most commonly used within a professional enviroment would be "Best regards"

I'm going to disagree with some of the other comments, and say that I almost always use "Best wishes," in my business communication.

I would never use "Sincerely," unless I was writing a formal letter, for example a complaint. 

When you are exchanging e-mail in a bussiness seting you could use any of "Kind Regards", or "Best Regards" .  In my opinion one should do that with both e-mails directed in or outside your company.