with any of

Is a legit carry for support. One of the few, you will nearly always do the most damage in the game( yes you will out damage Yi, Zed, leblanc often by 5-10k damage), you have a 70% 2 second slow on a 4 second cool down that can hit 3+ people, you have a ranged knock up, and you do %HP damage, so you don't typically need any pen till rather late. You also need next to no gold to be good, but since you have your W you can clear a wave and defend towers easily so you can collect those big fat bot lane waves in low elo when your team runs off and leaves the support sitting there trying to keep a tower for getting killed.
Against squishy supports you can max q first and chunk people, against annoying tanky or pull champs, you can max W and shred them. The %HP combined with thunder lords is enough to cause some serious pain to even the most tanky champs. Play back and use your E to peel your carry in mid game team fights then your ult to kill who ever dove them.

Those combos also have insane amounts of damage with the ults, once you hit 6 in bot lane if you and your ADC are in sync, one solid E knock up pretty much means the deletion of the enemy bot lane if you both ult and hit.
Other supports that can potentially hard carry are:
but Zyra has less HP than a lvl 0 poro and can get deleted, Anivia takes skill to not screw over your team with walls and doesn't really engage so it's hard to carry with her, and Voli is a way under played support but has some serious weakness to certain champ combos in bot lane.