It is quite a good question, actually.
You can check the Order Management for some small 3-Layer Architecture example:

For a Case Management Framework example, they always give the OutTracker by OutSystems. It also follows a nice 3-Layer Architecture structure:

Pretty much every application from any of the OutSystems' teams should be fine. As far as I have seen most of them follow the best practices most of the time ;)
A lot more advanced is the Application Framework set of applications (Reactive and Traditional Web) by @Justin James. However, this is more for the boilerplate around the actual application, and not a full-blown sample application.

You can check his articles on Medium, along with those of @Leonardo Fernandes, @Remco Dekkinga and the other guys - some high quality content.
https://medium.com/search/publications?q=outsystems&source=search_post----------------------------------------
BTW you can set a tick on Apps on the Forge and see what the top downloaded results are. Haven't gone through most of them, but I am pretty much sure, they are all fine:

Good luck ;)