You're using which as a noun whereas the problem uses it as a determiner or noun modifier
"Which is the most bitter wine?" should also be acceptable. In fact, though, "most bitter" is less common than "bitterest".
"Which is the most bitter wine?" and "Which wine is the most bitter?" are different questions. Imagine there are two boxes which you can't see through, and one of them has the bitterest wine in it and the other has the sweetest honey. Now you would ask "Which is the most bitter wine?", but not "Which wine is the most bitter?"
That possibly depends on where you're from. Personally I prefer most bitter.
Does Korean have varied intensifiers like English?
Like how we often use more/most but sometimes we use the -er -est suffixes.
Or does Korean stick to just using 가장?