It's a really tough choice and wholly depends on my mood. If I want a straight up blues rocker the first album is the go to, if I am in an acoustic mood III is where it's at but I have to say for all around catalog of all their styles the album I keep going back to again and again is Physical Graffiti.
I think that album is my favorite all around Zeppelin slab.
amenLed Zeppelin IV gets my vote - it's a perfect synthesis of the best material from the first three albums yet not short of originality from the band by any means (I know of course that using originality in the same paragraph as Led Zeppelin is contentious. I am not that bothered about the plagiarism personally as I know that Zeppelin by and large loved the music they 'borrowed' from and I feel that they did so to create art rather than simply to make easy money, just wanted to say that). Four Sticks is a very good song with an interesting and very cool guitar riff, the other seven are all terrific. It is an exceptional album by any standards. I am one of those who loves Stairway yet I also greatly love When The Levee Breaks, Black Dog, Going To California etc. One song I've been loving lately is The Battle Of Evermore, it's extremely powerful for such a stripped back song and Plant's vocals, especially on the repeated 'Bring it back' section, are awesome.
I'd put Led Zeppelin II second simply because I can put that album on and enjoy every minute of it from start to finish, there isn't a bad track on that one and it's got several extremely memorable guitar riffs. Physical Graffiti edges out Led Zeppelin I for third place on the grounds that it is an extremely strong album (and therefore even stronger considering it's a double album). Disc one of the album is fantastic throughout, with Custard Pie being a perfect starter and Kashmir a perfect way to end. I love a lot of disc two as well - In The Light, Bron-Yr-Aur, Ten Years Gone and The Wanton Song especially, and unusually for a double album there's no obvious filler. Night Flight is probably the song I care for least and I don't consider it bad.
I don't think you can go wrong with this band on anything from Led Zeppelin I through to Physical Graffiti - I even like The Crunge and D'yer Mak'er, and let's not forget Immigrant Song's superb B-side Hey Hey, What Can I Do fits in this time period too. There was a drop in quality on Presence though - out of all the songs I think only Achilles' Last Stand is definitely good enough to go on Physical Graffiti I think, although Nobody's Fault But Mine, the underrated For Your Life and at a slight push Tea For One are also quite strong and would also stand a chance of making the second disc. The other three songs are not quite up to the band's previous standards IMO.
I've realised I've gone on quite long but as you probably know I am a big Zeppelin fan and love so much of their music so I got a bit carried away.