All of them are important for me if you want me to read a fic in the first place, but what essentially decides whether or not I choose to upvote is the grammar. If the plot is awesome and the characters are perfect but the grammar is lacking, I'll grow tired of a story. I might read it, but I won't upvote.

I'd say all three.
Basic grammar skills are essential. I usually drop the fics if they're really bad in grammar.
To be honest, it's super hard to find good writing quality where the author uses a little more difficult vocabulary and syntax (more varied versions of sentences, not just simple ones). So I'd usually go with ok grammar, readable ones.
Quality story: Definitely. If I feel like the story has a very good plot, and I've truly enjoyed it: it gets an upvote.
Quality cast: In my opinion, to get a good quality story, it has to have good characters first.
These three things are all added into an upvote, in my opinion.

A perfect grammar doesn't create a perfect story. Sometimes, I stumbled around a few fics with perfect grammar, vocabulary and structure but the plot is very predictable and there's nothing special about it. I think three of them needed to be balanced well. The grammar doesn't have to be perfect as long as the author knows the rules of grammar, the characters needed to be described nicely, and the plot doesn't need to have a lot of plot twists. Doesn't matter if a 13 years old wrote it, as long as the story have those three things balanced.

intersting plot so i cant stop reading and feel involved in the story
characters are interesting and complex and a little ambiguous so they feel real
grammar and structure is good
interesting plot and plot twists
I finished the story haha
Ive upvoted stories that have bad grammar since many authors arent native speakers. as long as the story is interesting

The writer has to deliver the plot and ideas of the story well. That's the main thing for me. Even if there are mistakes here and there, if the story speaks out to me or has left an impression on me, I will give it an upvote.

I go with Others as for me to give an Upvote it has to have
1. Quality Writing
2. Quality Story
3. Quality Cast

As of one without the other is mostly uninteresting to me.
I might read stories that has a good story with just an OK grammar, but definitely won't give an upvote.
As of Quality Cast... if you got good story, the cast (Characters) are good as well as how Characters development is one of essence of the story.

Well that's my own opinion though

I think A, B & C have the same importance but it depends for the plot of each story, at least for me.
For example if I'm going to be reading some , I know it mostly never has a story/meaning, but I do like reading with good writing and use of words, not like if a damn 12 year old wrote it. orz

I answered A, because that is essential if you wish to keep me interested in a fic. However, B and C are important as well; perfect writing can only do so much if the plot is, eh, ty, and the characters are boring. Thus, I guess, D. Pfft.