I just wanted to go through my thought process when I'm picking a support in Draft Mode.
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- Enemy team has an AOE ult comp. Ex: Fiddlesticks, Gangplank, Kennen, vladmir, Amumu, Morgana, Sona, Galio. If necessary buy a Quicksilver Sash so you can pop out of stuns and reset the teamfight, and now the enemy team has blown all their cooldowns!
- Enemy team relies on suppress assassination. Ex: Malzahar, Warwick. Tornado to win!
- Your carry has good poke in lane (shield them for super poke). Ex: Caitlyn, Vayne, Ezreal
- You are laning vs Alistar or Taric. A personal opinion, but I find Janna quite effective vs. these supports. They rely on getting up close and crowd controlling your carry, but your shield protects the carry from damage when you see them moving in to knockup / stun, and your tornado disrupts their combos.
- Your team consists of champions with low mobility. Janna's Tornado / Ult can help carries and mages with no escapes from being assassinated. Janna excels in kiting and chasing.
- Janna is the least item dependent support in the game. Take advantage of this buy buying every single ward (and lots of them) and telling your teammates not to, increasing their farm by a little.
- You are laning with Caitlyn. She can wreck almost any lane with infinite mana / heals.
- Your lane mate is a carry that needs tons of farm to be effective, and therefore will not do much early game. You just need to be a good babysitter. Ex: Ashe, Kog'Maw
- You are laning against Vayne. Vayne relies on combos for damage. Heal your carry after the first hit and Vayne's damage is substantially decreased from the armor buff.
- You are laning against Taric. You will easily out sustain him, and prevent most of his kills since his burst damage isn't high enough.
- You are laning against a high poke damage support. Ex: Zilean, Kayle
- You want to make sure an enemy Karthus doesn't snowball off your team's near escapes.
- Your team consists of multiple "squishy" champions. Soraka's burst heal can really keep these guys alive when they get burst by mages/ assassins.
- Your team is an AOE ult comp. Sona's ult can facilitate landing other's ults, or extend the durations of AOE stuns. Ex: Fiddlesticks, Gangplank, Kennen, vladmir, Amumu, Morgana, Sona, Galio. Sona also facilitates these team comp's because of her AOE speed buff that will help initiation and chasing after initial damage is done.
- Your team consists of multiple off tanks / tanks. Sona excels in sustain healing. Champions that can take an initial beating will benefit more from Sona's heals than squishies.
- Your lane mate can take advantage of your ult once they're 6. Ex: Miss Fortune, Graves, Ezreal
- You will be laning against a support / carry with consistent poke, but not as much burst power. Ex: Ezreal, Caitlyn
- You plan on early game aggression with your carry. Ex: Tristana, Vayne. Taric is all about winning lane.
- Their team is 4/5 physical damage (armor buff ftw). Keep in mind that most teams are at least 3/5 physical damage at the moment.
- You aren't laning against Soraka. She will easily out sustain you and prevent kills. Taric's damage isn't very high.
- You plan on early game aggression. Alistar has high base damage and his headbutt/ knockup combo is very strong early game. Good lanemates: Vayne (pin), Tristana (slow/heal debuff), Ashe (slow/crit).
- You are laning against a good poker, but not burster. Ex: Caitlyn, Ezreal. Alistar's heal can easily keep up with poke, but not burst.
- The enemy team is relying on suppress assassination (headbutt ftw!). Ex: Malzahar, warwick
- Alistar is a good counter to Galio. Ult + knockup or headbutt will cancel his ult.
- Alistar is a good pick vs a team with only 1 AOE ulti, but not multiple. If Fiddlesticks CAWCAWCAW's you from over a wall, a quick headbutt will put him back in his place, but if you have multiple aoe ulti's you have a problem.
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Well these are the thoughts that run through my head when I'm thinking of which support to play. I have only outlined these 5 because they are the most viable / popular for ranked play at the moment, and quite frankly making a list of every support-esque character would be quite long. :P I hope you learned something by reading this.