My favoriate animal(s) (is/are) (a/the) cat(s).

1. My favorite animal is a cat.
2. My favorite animal is the cat.
3. My favorite animal is cats.
4. My favorite animals are cats.

I am inclined to use No. 2. But I think I have seen all the patters above. Which sounds better?
A similar problem I have now is:

5. He looked up the meaning of the words.
6. He looked up the meanings of the words.Both versions seem correct to me, but I wonder if there is any shade of difference between the two. Any opinion?
Eamer