You're overthinking this - and focusing on the wrong aspect of the cover letter.
There is no magic opening line or phrase that's going to get you the job, people are generally looking to hire someone who can do the job not someone who writes a gimmicky cover letter after mainlining a thesaurus.
Concentrate on making sure that your cover letter and CV/...
"To Whom it May Concern" covers all cases.
Even if you think you know the name of the hiring manager, or who the supervisor will be, or who the HR person filtering the applications is; they are not the only ones who will be reading the cover letter.
Every time I was the person who filtered the applications I read all the cover letters. I did the same thing ...
The hiring manager, not HR. You'll get priority from HR if you do that. At least, that's the way it worked when I worked for HR.
There was a little checkbox in the database for that in our database after I scanned in the resume. That checkbox insured the resume would appear on top of the other resumes with the hiring manager. The paper copy would also be ...
When in doubt:
To Whom it May Concern,
Unless you are absolutely sure of all parties, this the standard opening to any formal correspondence. This covers everyone, known or not, or those who might view it in the future.