As an introductory statement,
私は想像力が貧しい sounds the most natural in the first set.
私の想像力は貧しい is OK, but it is about 私の想像力 and this might sound a bit sudden unless you have been talking about someone’s imagination till that point.
私の想像力が貧しい sounds like you are saying “It is my imagination (not anything else) that is poor”.
In the second set,
私の車は赤いです sounds the most natural. It is about 私の車 and rightly so.
私の車が赤いです has the same problem as
私は車が赤いです sounds weird as an introductory statement about the color of your car. It is about 私 in contrast with another person who may have a car of a different color or some other thing that is red.