Epic, shawl, and my PoM boots all took a huge amount of time to get, but really I'd have to say my (six)
Driftwood Treasure Chests, just because they're kind of unique but also (virtually) "best in slot" (10 slot, 100% reduction, and while their weight isn't BiS, I don't really care because self-buffed I have more than enough strength to carry them).
And no one else I've ever met has six of them ... in fact very few people have even one, as they're a giant PITA (roughly 100 kills each are required, at 22 min respawn).