Each of the REST clients have their own advantages and limitations. So far, RestTemplate has proved very popular, but looking at the Spring future plans and introduction of WebClient makes it less demanding. A migration to WebClient seems like a better choice.
OpenFeign is very sleek and intuitive. However, in the past, lots of Common Vulnerabilities and Exposures (CVEs) make it vulnerable and so the least preferable choice. It also depends on lots of third-party libraries. This is where most of the CVEs were reported.
Java 11's HttpClient looks very attractive and provides lots of advanced features. If you can grab it and write an intuitive API on top of it, then it looks like the best suited choice.
We have discussed the pros and cons of each of these REST client options. You need to have a hard look and adopt one of these, or many other available REST clients...