Comments for sure! It lets me know that the person is actually reading the story, and as writers, the thing we want most is readers. If I could confirm somehow that views and likes counted as reads, I might be fine with them, but there are reasons why a person could be liking the story that have nothing to do with whether they actually read it. And views of course in no way count as reads.
I'm not a ghost subscribers at all. Mostly because I know how frustrating it is when someone is a ghost subscriber. Like, for pete's sake how much effort can it really take to pause for a moment, remember to leave a like, and leave a nice comment? Especially when leaving one or not can really influence whether a creator wants to keep making their story. The only time when I'm a ghost subscriber is when I bookmark something that I really want to read, and I just haven't gotten to it, yet (one of the reasons why I try not to get someone's hopes up by subbing willy-nilly).
Hmmm... I like subs, but subs are some of the most frustrating things on this site. I swear that there's a bug where, whenever I update, someone unsubscribes but then for some reason I get a new sub on the same day. There was a weird instance of that lately where someone had subscribed, and then a few days later, they subscribed again. I was just like, "Wait a minute, didn't that person already subscribe?" LOL