It sounds very awkward to me. "In" is the wrong preposition here. Options: number one choice throughout the world. Number one choice around the world. However, something like this is usually phased something like this: It's chosen by more people worldwide. Worldwide, more people choose it.

It sounds ok, but maybe a little awkward. You might say "it is the first choice" instead of "number one choice."

That is perfectly clear to me. Yes,nth at is a good endorsement for a product.