I remembered this topic when I was listening to some late era Led Zeppelin today, I think I've got a decent candidate for an album that I think is somewhat one-sided. Although it is perhaps debatable as to whether it counts as 'proper album', I nominate Coda as I feel there is a significant disparity between the two sides, with side one being my favourite by far.
Side One
1. I'm Gonna Groove
2. Poor Tom
3. I Can't Quit You Baby (Sound Check Version)
4. Walter's Walk
I went for Coda because I think side one is actually pretty awesome - We're Gonna Groove is classic Led Zeppelin blues rock, Poor Tom is a great, quirky little song which cleverly mixes folk, blues and rock influences (and Bonzo's drumming is great). I Can't Quit You Baby was already a very good song and if anything I like this version better than the one on LZ I, and Walter's Walk is another strong, powerful song and wouldn't have been out of place had it been included on Physical Graffiti.
Side two, by contrast, consists of three outtakes from In Through The Out Door, none of which I consider to be particularly impressive, and Bonzo's Montruex which is little more than an extended drum solo. To me it feels like side one is a reminder of how immense LZ were in the early days, whereas side two feels like scraping the barrel a little. The end result is that I listen to the former side a fair bit but largely ignore the latter.
I did consider nominating Presence, where my three favourite tracks are all from side one (Achilles' Last Stand, For Your Life and Nobody's Fault But Mine), but Tea For One gives side two I find to be just about good enough to make it less unbalanced, and I consider Royal Orleans and Candy Store Rock to be about equal in quality.