The black war tank isn't more prestigious than any other pure time investment item. Artificial limitation does not create prestige. Yes, it's cool when you see it, but that is not prestige. It's novelty.
OT: The only mounts that actually carry prestige are the current cutting edge mount and the gladiator mounts. Those actually required you to challenge yourself in a way more than just throwing time at it.
As far as desirable goes? Ask 10 people, and you'll get 11 answers.
Iconic? I dunno. That's a weird question to answer. It really has nothing to do with rarity. It would probably be someone's mount. Best answer would probably be Al'ar purely because of how obnoxious it is.