If we go by what are the rarest and coolest mounts I would say, Obsidiannightwing from Recruit a Friend, Obsidianworldbreaker from the 15th Anniversary, the corehound from the 10th Anniversary, the Hordechopper from the promotion, the uncorrupted voidwing from N’Zoth, the airship mounts from Blizzcon and the mounts from the Alterac event.
If I go what mounts I cherish the most are uncorrupted Voidwing & the Frostwolf from the Alterac for Kraktor, the traveling urtum and the Teldrassil Hippogryph for my Druid and my Felsteed and black drake for my Warlock.