I'm not a statistics person, so I can't give you the actual sensible numbers, but I think it's not that any one of those is most important so much as they're interconnected. You will have more views than subs, because not everyone who looks at your comic is going to be interested. Ideally you're converting as many views to subs as you can, but as long as you're getting a decent percent and your number is growing in a natural way, you're doing pretty good. Not everyone who subs and reads is going to comment. Engagement is probably the most personally rewarding kind of interaction you can get, but there is an actual statistical number out there somewhere that represents a good percentage of engagement.
There's a balance to how concerned one should be about any of these numbers. I tend to feel like novice creators get worked up really easily by lack of growth. But being aware of numbers and willing to learn can help you figure out where your issues lay. If your views are low, you need a more eye catching thumbnail. If your subs to views is really low, the first impression of your comic/story might not be good enough, which could be a problem with the art level and/or description. If you have lots of views and subs but really low engagement, you might need to address how you present yourself and how to communicate better with your audience.
So it's good to know the numbers and have some awareness of where one might need to improve. But I think it's healthier and more important to have a "doing it for myself!" mindset, because that really helps you move forward early on. Being hung up on the numbers tends to be demotivating.
Also going to toss out there that people using tapas generally have accounts and sub if they're reading a comic, where as on youtube, you may just regularly tune in to certain channels without subbing. Thus views and interaction have a higher meaning. Views on youtube are people actively watching content, views on Tapas (that don't convert to subs) are people looking, deciding they're not into it, and leaving. More or less.