I’ve tested rtorrent using the rpc and the httprpc plugins in ruTorrent and both have worked for me. Sonarr successfully added a torrent and imported it after completion, using each plugin. Note that you should only have one of these plugins installed at a time, never both.
The host and port settings are for your web server that is hosting rutorrent. So if it’s on the same box as Sonarr, this will usually work (depending on your web server setup): localhost and port 80.
The unique part for the plugins is the URL Base. For the RPC and HTTPRPC plugins respectively, use these paths (modified if you set up your rutorrent in a different web path):
/rutorrent/plugins/rpc/rpc.php
/rutorrent/plugins/httprpc/action.php