There's stuff like We R Are Why and Quadrange which I've never owned, same with the collabs. But sticking with the 'core' albums and EPs:
I've only listened to L-Event once (it got sold in a 'shit I need some cash' panic just after I bought it, but finally another is in the post as we speak);
elseq I'm still waiting to put together a nice 5CD box set before I give it more than a skip through (I gotta browse my shelves and find shit to enjoy it - weird like that);
I've probably not listened to Oversteps and Move of Ten that much, because they came out at a time when I was heavily into older analogue sounding stuff and so they never had the opportunity to work their way into my head;
I always found EP7 really unsatisfying, so it's one of the few records I skip through, to tracks like pir and dropp;
Gantz Graf other than the title track.
Untilted is my least favourite record (combination of being their least melodic, and the loooooong tracks), but I've listened to it quite a lot, trying to get into it as it's fascinatingly alien to me.