This is the official f-droid client ! The UI is a bit old comparing to new standard but it is not ugly either. The UX is not that great as it can't recommend you apps and the search is quite terrible. However it's the stock store on CalyxOS and DivestOS so it comes with installation privilege and let you install apps in the background. So if your OS comes with the official client by default I recommend you to stay with this one. However if you want a different experience here are the third party client I recommend you to try.