Houses Of The Holy is my favourite Zeppelin record, their debut and IV are also strong contenders. Frankly their catalog is pretty darn strong as a whole. However Houses has some absolutely amazing tracks, The Song Remains The Same is a phenomenal opener, The Rain Song is majestic and peaceful. No Quarter a progressive and eerie track. The Ocean a good rocker to close the album. It is strong from start to finish.
Zeppelin were my first #1 rock band, the one I learnt all discography by heart, talked about with my classmates, the first real love. I was saving up from my allowance to buy the 2003 DVD when it came out.
I still like and respect them, but over time not only has my general musical taste kinda shifted, but most of their albums seem uneven now (at least two to three songs on each I really don't get why they're there) and I have built up an allergy for Plant's vocals. There's just something to his tembre and general vocal approach that makes him particularly obnoxious (my wife said the same after we player IV on a car trip immediately followed by Made in Japan, where Gillan is doing his best to be somewhat obnoxious as well, yet his voice is sooooo less grating and more pleasant to the ear).
My fave remains PG, my very definitely favourite song would be Achilles.
I'm currently having some trouble trying to choose between the original vinyl version and the 2014 remaster of Led Zeppelin IV. Luckily, I don't have to worry about the logistics - I have 320 kbps MP3 rips!
Edit: The original seems to have a much warmer sound as compared to the remaster's stark artificiality. It's also easier to play at higher volumes. All I have to do now is test it on a stereo system.