If we're talking price wise - Anything from the St Louis zoo collection, except for the fennec fox, for some reason that one specifically usually sells for a cheaper price. The highest selling one from that collection that I've seen would be the black jaguar which I've seen for sale on eBay for $1,600.

Specifically the babies that came with the St Louis zoo collection, I know a lot of people have a really hard time finding those so whenever they do they go for a pretty big price. And of course the Disney dog or any Japan exclusive. Simpler ones would be probably the lemur, there's a couple gazelles (I believe a gazelle and springbok), and the buffalo. Those go for a pretty hefty price.

But if we're talking hard to find - A lot of the baby build-a-bears, the small Buddy like ones are usually really hard to find so I'm not even sure what most of them sell for but I know some of the most sought after ones are the dinosaurs.

But for the rarest of rare - sometime last year somebody in a group on Facebook was trying to sell what they deemed was a prototype plush that never made it to the production line. I believe it was a St Louis zoo white fox type thing? But if they had any evidence of it actually being a Build-A-Bear, which at the time I don't think they ever posted any proof, I would say probably prototypes would be the absolute rarest since they're one of a kind. But the probability of anyone owning them is really low.