Any reason "어느 와인은 제일 씁니까?" shouldn't be accepted?
The wine itself is what needs to be determined; the speaker is not asking something where a specific kind of wine is given as a topic.
Is it possible to write thus as: "와인이 어느 가장 씁니까?"?
No thats not possible. The question word must go before the subject.
Not really, it sounds a bit odd. HOWEVER, you could say 와인은 어느 것이 가장 씁니까 I believe...
어느 should be first of sentence
Taste and maybe you'll find out
가장 쓴 와인은 무엇입니까?