कोनसा(masc. sing.)/कौनसी(fem.)/कौनसे(masc. pl.) is the actual word for 'Which'.
किस is the oblique form of क्या (what) and कौन (who) which is used when they are objects of a postposition.
When कोनसा/कौनसी/कौनसे appears as the object of a postposition, you can either use its own oblique forms कौनसे(m)/कौनसी(f) or किस and the meaning is pretty much the same. This is similar to how, in English, you can replace 'Which' in many questions with 'What'.
So, you can say both 'राज कौनसे घर में रहता है' and 'राज किस घर में रहता है'. Usually, the former is used when you have a given set of options of which you are asking one to be chosen and the latter is used when the question is more general but this is not a rule.