Well, I think the answer is axiomatic! The answer is - musically - C major. Because most starter instruments start with C - and is technically the equivalent of all the white notes on the piano, for example.
As you progress you build a ‘library’ in your head of all the other ‘accidentals’ - the equivalent of the black keys - which form the sharps and flats in the other keys, both major and minor.
By learning the C Major notes - you are also subconsciously playing the seven 'modal keys starting on C - The seven modes are named: Ionian, Dorian, Phrygian, Lydian, Mixolydian, Aeolian and Locrian.
You must now try and pick yourself up from the floor!
But you don’t need to know this - but playing in C everything fits in other modes later, for example mixolydian is used in Rock and Roll - Dorian for Jazz, with Irish music using Ionian, Aeolian, Dorian, and Mixolydian!
I’m only telling you this because your understanding will be much easier if you begin in a non-complicated key that C major is, with its equivalent minor - A minor
Yes - others will tell you that all these modes exist using other keys as starting points. But many music teachers will tell you that a C Major starting point is the best uncluttered start - and believe me - I’m no teacher!
Starting on C Major will inform your learning if ever you want to play other instruments - and you will! As your base knowledge and tune ‘library’ will be transferable to another instrument - making your learning faculty faster and faster as you progress.
Remember music is only a code - like computer coding. You just get to know it as you use it. And it will become obvious and understandable - as you practice and play and develop muscle memory. Much of it subconsciously.
Music will grab your soul and emotions when you are seized with the sheer joy of converting black dots into music!
It will provide you with more spiritual calmness and pleasure than, say, football can!!!
Start off on a C Major Chromatic - you’ll be able to play any key - just initially remember: tone tone semitone, tone tone tone semitone.
And you’ll soon find out that that is the code for any key signature. If you don’t already. Just chill - and the penny will soon drop!
The important thing at the moment is getting the best fluency with the harmonica - you’ll develop the sound - and pitch - later. You’ll also have the best range for bending notes of the Chromatic. Andy will tell you that the lower the range - the more difficult it is to bend the notes. The middle range of C will develop the most flexible strategy for bending anything later! Tell him Andy!
Best of luck!