Wow that's.... kind of a broad subject.
There really isn't just one thing that makes something "great". It's ultimately a combination of all the parts coming together, but then there's also an element of personal taste or appeal there too. Sometimes a work can have such strong personal appeal that somebody can love it and want to experience it over and over even if a critic would insist it's "objectively bad" (Twilight is a great example), and sometimes it can be so inaccessible that people don't want to watch it even though it's considered one of the finest works ever created (Hamlet starring Laurence Olivier).
Characteristics I'd personally look say make a great work would be:
- Thematic consistency. The work knows what it is trying to say and every element of it reinforces that theme. For example, in Fullmetal Alchemist, the theme is "equivalent exchange" ie. If you want something, you need to give or sacrifice something of equal value, and if you try to break that rule, the universe will take something of value from you in some way. This theme consistently affects every character and story in the comic and it ultimately concludes that you simply can't have something without sacrificing something else, so sacrificing your own time and hard work is better than sacrificing other peoples' wellbeing. Another good example is Enter the Spider-Verse, with its clear theme of "anyone can be Spider-man." showing a bunch of characters who all have different ways of being Spider-man, and the thing that unites them all ultimately is believing that with power there is a responsibility to use it to help others, but you have to be Spider-man in your own way rather than trying to be somebody you're not.
- Polished design and production: Whether it's a comic drawn in a simple line art style or a big CGI-heavy Hollywood movie, the aesthetic should feel consistent and deliberate, supporting the overall atmosphere of the work and with small details that add to the immersion rather than pulling the audience out. There are tons of great examples of this, like the Lord of the Rings films, Wallace and Gromit. In comics, a great example would be Watchmen, where every element of the style and design, and even the covers add storytelling and depth. Obviously in comics this also means a clear, readable font that fits the tone, and attention to elements like gutters, the logo/title font and how they support the overall "Brand identity" of the work. Overall, it's the sense that there was a broader creative "vision" behind it and careful attention to detail across every aspect of the tone, setting and even how it's marketed.
- Great Performance or Expression: The words expressed through the writing, the feelings expressed through the drawing or animation, or how the actors perform the words and emotions, how it's all framed, just the general artistry of the work should obviously be excellent. Shakespeare wrote some sublimely beautiful words, which when spoken by an actor really giving it their all through body language, facial expression, tone of voice and timing can create a magical experience, and I think a great comic can be like that too, when you have punchy, effective words accompanied by a drawings that really hit on the emotions the artist wanted to get across, it's just fantastic.
