Could you tell me which was your favourite subject ?这句话是否正确

Could you tell me which is your favourite subject ?

Could在这里不是过去时，是婉转语气，所以要用is

could
v.aux.
1. can的过去式
She could play the violin when she was five.
她五岁时便能拉小提琴了。
2. (用于虚拟语气,表示与事实相反的设想)能,可以
I wish I could go to France.
但愿我能去法国。
I would go if I could.
能去我就去。
3. (表示可能性)可能
That could be my train.
那可能是我要乘的火车。
4. (用于婉转语气)能,可以
Could I smoke here?
我可以在这里抽烟吗?
Couldn't you try it again?
你可否再试一下?

正确

问的是过去所喜爱的subject,单现在已经没兴趣了

这里体现时态的是was,如果句子中加上one year ago等表现过去时态的时间状语则时态更准确