This is actually a very interesting point that I've come across on a few different platforms recently (Patreon, for example, [also targets their homepage to creators, not consumers](
https://www.patreon.com/)). I wonder if Substack just doesn't want to deal with opening the door to some newsletters getting "free promotion" by being features on their homepage? Nonetheless, I'd love to see "trending" or "editors picks" or something.
Nonetheless, here are some I subscribe to:
Big, by Matt Stoller (about monopolies): https://mattstoller.substack.com/
Margins (about business and tech): https://themargins.substack.com/
Flow State (music): https://flowstate.substack.com/
Normcore tech (tech, data science): https://vicki.substack.com/
The Passion Economy (business): https://passion.substack.com/