Exterior paneling which?

[Anemon86]

What is the best timber, panel wood for outdoor use, for a shed door?

This is what the door looks like today. We will keep the frame, but need to replace the wood on the outside.

 

[FredrikR]

What is the best timber, panel timber for outdoor use, for a storage door?

Best in what way?

You mentioned storage door, is it like a wood shed or an insulated space?
What does the rest of the storage look like?

<Edit>
Now I see an image has been added.
Choose something that matches the rest of the paneling, mount it vertically with some type of protection for the end grain at the top, possibly a metal sheet. Ensure all wood is painted on all sides.

 

[Anemon86]

Best in what way?

You mention storage door, is it like a woodshed or an insulated space?
What does the rest of the storage look like?

Bike shed that we have built. Not insulated. Made like a stable door type with a Z on the back