Exhaust:As of the preseason balance changes it was buffed. Post preseason it was absolutely the go-to choice, and is just as viable as before if not more than ever.
V1.0.0.152: http://leagueoflegends.wikia.com/wiki/Exhaust
- Now reduces attack speed by 50%.
- Movement speed reduction reduced to 30% from 40%.
- Now reduces all damage dealt by 30% from 70% for attacks and 30% from abilities/item.
When:There are many opportunities for you to use it, but when is the best time? Generally speaking in my experience you should save it for baiting, or overtly aggressive opponents. The slow is a great tool for when they run, but you should primarily be focused on reducing the damage by as much as possible. So before a Graves can Q-R combo you in-lane, remember. Or if a Leona is preparing you for her ADC, take note.
If the enemy team has a heavy gap-closer like Jax's Leapstrike, Irelia's Bladesurge, or Xin Zhao's Audacious Charge your AD Carry absolutely NEEDS you to exhaust said champion or your ADC will die immediately and you're most certain to lose the team fight.
Why:- Because its the best and scales the best into late game out of your current selection of summoner spells - even speaking of unconventional ones. It may be single target, but you can't put a price on how much damage it reduces early to late game. Only growing in numbers as the game progresses of course.
- If you don't have a good front line or defensive team, this is what you NEED. Even if you do for example, it still is helpful in the case that an enemy carry got fed and gives potential to turn things around - more specially for instance in YOUR lane.
Clairvoyance:Using Clairvoyance effectively requires experience with the jungle timers. You absolutely need to know both when and where to use it or else you're simply going to waste it on air that could have been a champion, or a buff. Overall while it can be useful, Exhaust should be picked above it unless you're practicing a rehearsed strategy. Not for solo-queue.
V1.0.0.152:
- Duration increased to 5 seconds from 4.
- Cooldown reduced to 60 seconds from 70.
- Radius reduced to 1200 from 1400.
- Summoner's Insight now maintains sight on units revealed by Clairvoyance for 5 seconds instead of extending the duration by 2 seconds.
When:- Generally speaking if you're playing support correctly you shouldn't need to use it on River apart from Dragon and Baron, assuming its warding and you're safe anyways from enemy champions trying to jump.
- Use it constantly to reveal the enemy buff timers for your jungler and team. Having the buff reset on the map, or being able to invade with vision at that time helps a lot for succeeding in invading or safely escaping.
- Using it more aggressively can be a great tool for allies in all lanes when the enemy tries to use the bushes to their advantage. For that reason I greatly wish you got assists for revealing them, but that day will likely never come.
Why:The main thing about Clairvoyance is that its a Jungler's worst nightmare. Bot lane already is warded, and what? Now you're not only going to be impossible to jump - but you're going to make it even HARDER on him to get other lanes to?!
"THE FUCK DUDE" -Amumu Its effective purpose is to prevent laning phase from being dominated by the jungler, and protected buffs/objectives. Otherwise its use it too situational and serves no purpose for teamfights, which is why its never sees the light of day.
Heal:The Health per level was reduced, but with the massive 50% more health being added its nothing but a scratch. All changes in mind, its stronger in team fights and in-lane.
V1.0.0.152:
- Health restored per level reduced to 15 from 25
- Amount allies are healed for increased to 100% from 50%
- Improved Heal now increases your champion's max Health by 5 per level instead of increasing the Heal amount by 10%
- Cooldown increased to 300 seconds from 270
When:- The time for you to choose it isn't really when you need to specify, but when not to pick it is all you need. Tristana or Miss Fortune already picked? Don't even think about it. That's essentially all that's to it.
- Baiting is what this thing is made for. Most moments are obvious, but you need to be prepared for. You're going to be in a risky situation and if you wif it, you're fucked and likely down a summoner spell.
- Teamfights, Teamfights, Teamfights. The most effective time that you can EVER use it throughout the entire game is when you're around all 4 of your allies. It won't counter a full AoE team, but consider how useful it will be to give your whole team 400-500 health with an Ancient Bulwark, and Taric Passive already saving your team that much effective health, etc. It does work.
Why:The primary reason you should roll this on a support is if you want to bait. Effectively it can counter specific ultimates in specific cases like Caitlyn or Karthus - but generally it doesn't do much else. Its very situational, but even so it can be very useful in-lane as its very underestimated since its quite underused.
Just as much as Exhaust, it does scale into late-game, but if your Jungler or Mid for instance pick Exhaust this is a very syngergetic summoner spell to pick up for your team on the flip side. Its strong in lane as surviving, but not as effective at securing kills. But you should learn to adjust your lane to this anyways.
For example if you're Taric and Stun-Combo them, it won't make much of a difference if you don't have Exhaust to slow them anymore. You'll still dominate the lane. Just scale even stronger into late game.
TL;DR:
Generally speaking, may very often be situational (hence this explanation of them all): Exhaust > Heal > Clairvoyance