ZetaTorrent is the best and can do everything you want !

It's probably better to set up Plex. I use ducky tv to auto download my shows and Plex to serve it to my tv. Plex can even auto update the library.

It's not torrents but Terrarium TV also has a download feature so you don't need to stream.

Forgot to mention, tTorrent has the feature you are looking for, selective download.

tTorrent is very good with remote and it also has web-ui feature.

how do you mean like select individual files from the torrent that you want? Or am I misunderstanding

but it doesn't work with remote, but only with mouse

This is why I use an Air Mouse , that & the stock remote mostly sucking IMO.

Granted, I don't torrent on my ATV devices, but still.

As for the file selection ability, there are plenty: Frostwire, zetaTorrent Pro & Flud, to name a few.

I hope you're not downloading Torrents on flash memory!

I use the GAIA Kodi addon and Real-Debrid. :)

I use Flud. You can select the files you want, plus it has the option for sequential download (so I can play the file while it is still downloading). Also it is a free torrent app without speed limits. It appears as an Android TV app and you can navigate with the remote, but it needs sideloading and you can't use some features like changing the torrents' order without a mouse.

EDIT: It doesn't appear as an Android TV app. I had used TV App Repo and I had forgotten it

What is the best app to stream/DL torrents as of August 2018 (with the option to select to stream/DL parts of a torrent).

Quasar/Kodi is not stable. Shame since it works great when it works. I need to be able to stream 15 GB 2160p MKVs. Something like TerrariumTV which by the way is not a torrent app is not relevant because the quality sucks :/