Favorite Led Zeppelin Album

I'm a big fan of III.  Definitely an underrated album in my opinion.  Celebration Day and Out On The Tiles are so awesome and yet rarely get mentioned in any lists of the best Zep songs.  Though, I do have to admit, Bron-Y-Aur Stomp and Hats Off To (Roy) Harper are likely the cause of why III isn't as highly regarded overall - not a strong way to end an otherwise great 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.