Your "top priority" is the thing that is most important to you. It's possible for your "top priority" to change depending on what is important at that time. If something needs to be done quickly, that can become the top priority until it's done:

Right now my top priority is getting this report finished.

It may be a little confusing, but we also use the phrase "highest priority". This phrase is used slightly differently. You use the phrase "highest priority" to talk about the thing that is most important to you in a more lng-term way:

Making sure that our customers are completely satisfied is our highest priority.