The entire Abruptum discography.
Having only heard a couple tracks of it on someone else's itunes, I can only imagine so.
Bucket Full of Teeth, a studio-only noisecoreish band with a bunch of dudes from Orchid in it, are pretty hard to get into on their full length
IV , but then again I liked it like two years ago, so maybe it's not so weird, since I still don't really 'get'
White 2.
Trout Mask Replica is a good bet for most inaccessible album that the majority of relatively informed rock fans have heard of.