In my heart, in my mind

I’d like to express something,
like “you are the number one in my heart”, or “you are the best in my mind”.
Is it okay to say that? Or it sounds weird?
I’m not sure the difference between “in my heart” and “in my mind”.

Thank you!!!

It does sound a little weird. A native speaker would probably invert the order of the clauses you have here.
Thus: In my heart, you’re number one. [no ‘the’]; In my mind [or ‘To my mind’], you’re the best.

I would say the difference between ‘in my heart’ and ‘in my mind’ is that in the former, you are expressing something that you deeply feel (possibly you love this person, or at least have some strong emotional connection to them), while in the latter you’re merely expressing something you believe intellectually.