Earlier in this thread I mentioned Sugar Gliders, but it was actually Feathertail Gliders I was thinking of (never been to a zoo that had Sugars on display). Anyways, I finally got to see these little guys in the flesh at Staten Island Zoo a couple of months back, so no longer on my "not-seen" list.
I agree with Maned Wolves either being a no-show or hidden. They were a no-show at National and Philadelphia and were barely visible in LA and San Diego when I visited; only at Audubon Zoo were they active and easily seen.
Another barely seen species for me is the Collared/Brown Lemur. In fact I only saw it in the flesh at a small rescue sanctuary despite the species being at a number of zoos that I've visited (Bronx, SD, National, Brevard etc).
Next time you're up here, try Turtle Back for the maned. They were active there each time I went by the yard over the course of 5 hours.