Favorite Led Zeppelin Album

Another vote for I

I, III and IV are tied for my favorites.

I could never pick a favorite. I love everything those four ever did, including pre and post Zeppelin. 

1. Physical Graffiti

2. IV

3. Houses Of The Holy

4. The Song Remains The Same

5. II

6. Presence

7. I

8. In Through The Out Door

9. III

No posthumous recordings considered. Hope the OP doesn't mind a complete list.

Physical Graffiti for sure.  LOVE How the West Was Won for a live album as well.