Taric- Taric's qualifications for this list can be summed up in two words: Ranged Stun. Though the rest of his abilities are great, I would still play Taric even if his one and only ability was Dazzle. His ability to interrupt, lock down, or halt an pursuing enemy make Taric difficult to deal with, even under good circumstances.
Taric is notably one of only two champions on this list with the ability to heal his allies. His ability to keep himself and his allies near full health throughout a long slog is invaluable, allowing him to trade damage with ease, and offsetting some of the drawbacks of his need to be close to the fray. Speaking of being close to the fray, that brings us to the next great thing about Taric: his auras. Like any paladin worth his salt, Taric has a number of auras that he grants to all nearby allies, making him a powerful presence in team fights. The bonus armor granted by Shatter is useful not only during laning (in which the primary mode of damage is AD based), but also during team fights, where he can activate it to help give his allies a small damage boost. Taric's powerful stun, sustain, and beneficial auras make him a great initiator and defender, as well as making him truly, truly outrageous.
Soraka- Everyone loves a heal bot. In the three years I have been playing League of Legends, I have never once heard someone say, "Ugh, SORAKA? Why would you pick her?" Know why? Because everyone likes full health and mana. Though her kit tends to lead to a somewhat passive play style, Soraka gives her carry the sustain they need to single-handedly deal with the threats of the enemy team. Couple that with a global heal, mana restoration, and a convenient silence, and you've got a force of loving harmony that can only be challenged by My Little Pony.
Though she isn't as flashy as many other champions, Soraka's presence in a game is definitely significant. Between her buffs to allied defenses, two powerful heals, and the ability to help sustain nearly forever, Soraka can help throughout the continuous trades of the laning phase, and with clutch heals during a team fight. Skilled Soraka players can also turn the tide of a duel going on far away with clever usage of her ultimate. Perhaps the truest support of them all, Soraka's ability to sustain and protect her allies makes her a worthy addition to any team.
Thresh- Though I personally am not a Thresh player, I am always happy to see the grim hook-master on my team. With multiple powerful CCs and the ability to quickly get his allies in or out of a fight, Thresh's greatest strength is in affecting positioning. Though he is an able fighter, Thresh excels at getting his allies out of and putting his enemies into harm's way.
Thresh's plethora of CC and repositioning skills are great for quickly turning the tables on an engagement or bailing out of a situation that doesn't go as planned. His ability to change quickly between offensive and defensive makes him difficult to predict and scary to face in lane. If played well, he can tie down an opponent while granting his allies defensive bonuses and giving them an easy escape route if things get hairy. Thresh's unique skill set and the ramp caused by Damnation make the Chain Warden a frightening opponent throughout all phases of the game, and a worthy addition to my top support picks.
Though many players don't choose to play support very often, the role is a crucial and rewarding one. The success of many games has been built on the backs of a willing support, and on the aid they lend to their allies. So next time you want to say, "Screw it, I'll just lock Clarity / Teleport Eve and hope someone drops", try one of these guys instead.