Never lose heart! What [the] most important is not the result ...

[cnjackie88]

Senctence 1: Never lose heart! What most important is not the result but whether you have tried your best.

Senctence 2: Never lose heart! What the most important is not the result but whether you have tried your best.

Which sentence is correct?

 

[Piscean]

Neither.

Never lose heart! What is most important is not the result but whether you have tried your best.

 

[Tarheel]

If I were you that's one English phrase I would unlearn. Either you do your best or you do not. The "try" is in the effort.

 

[emsr2d2]

Which sentence is correct?

Sentence 1: Never lose heart! What is most important is not the result but whether you have tried your best.
Sentence 2: Never lose heart! What the is most important is not the result but whether you have tried your best.

As you can see above, neither sentence was correct. Piscean gave you the correct version in post #2 too. If you want to use the definite article somewhere, you need to change the wording to "The most important thing is not the result ...".

Note that I have improved your thread title, moved your question to the beginning of your post, and corrected the spelling of "sentence".