First Lady of the Life Binder
Raid: Dragon Soul
Zone: Tanaris (Kalimdor)
Drop chance: 6.8% during the event, otherwise 1.8%

Dragon Soul is a raid that is rather poorly remembered by the community. Many did not find the “epic final battle” against Deathwing very good – there were way too many burning tentacles but very little “heroic battle against a dragon,” as many would have liked.
Nevertheless, Dragon Soul has a few of the cooler mounts, including the First Lady of the Life Binder, which drops from the final boss on heroic difficulty.
For fairness’ sake, it should be mentioned that Blizzard has significantly weakened some of the longest fights in Dragon Soul – such as the fight on the back of Deathwing or against Ultraxion. But the raid still has a fairly long duration, partly due to the “RP insertions” where characters converse for a while before it finally continues.
If you want to give your future self a little time, you should pass on the First Lady of the Life Binder – or take advantage of the event now to benefit from the significantly increased drop chance.
Of course, the question of which mount is worth farming is a very individual one. Tastes differ. But if you want to maximize your time savings and want your mount collection to be as filled as possible, you can do no wrong with the 3 mentioned mounts.
If you want to farm more current mounts – there are also plenty of them in The War Within.