Really it's a toss up between plot and characters, with probably just a slight emphasis on characters. Plot alone won't make me like something if I don't like the characters, but if I'm invested in the characters I'll keep reading, even if the plot isn't all that great. Proper spelling, and grammar is nice to have as well, but you can use proofreaders to fix that stuff - so the other two are the most important.
The plot and the characters. I could care less about the grammar, as long as I can understand what you're saying, then I'm good. The plot has to be really good. The writing style, regardless of grammar, has to be even better. Especially nowadays because today we have ideas that have been done way too much and if I read a book with a cliché idea it must have good writing.
I would say characters above plot. If the plot is thin but the characters are interesting, I will be invested in it and continue reading.
Grammar is subjective. If the character would not speak in grammatically correct tones, then it is necessary to adjust grammar. If the character says (da whole thang were a mess), then it would not make any sense to type the statement grammatically correct (the whole thing was a mess). Sometimes it is necessary to do it "wrong" ya know?
That is a difficult question to answer... I think they are all equal... What would the characters do without the plot? What would the book look like with no grammar? I don't think any of them have more value than one another
I'd agree with Sephira on this (as I do on many things). Characters just a tad ahead of plot.
Grammar? Well, of course, though one writer I enjoy is J.D.Robb (a pen name for Nora Roberts) and her paperback stories are full of errors. She could use a better proof-reader. but she's such a great story teller that Daizy and I forgive her and keep reading.
One thing that turns me off is the use of foul language. I don't mind it as dialogue (it has its place there if the character demands it) but if the writer starts using it in virtually any other form it's a signal for me to stop reading. It's like holding up a sign saying, "My vocabulary is so poor that I don't know how to express myself."
One other thing I find to be a turn-off (and remember, I'm older than God) is an overly-detailed sex scene. It's J.D.Robb's weakness and we skip through them. It's not because we're prudish -- my generation probably invented sex -- but because she can carry on for several pages, and they spoil the flow of the story.