This is for my W.I.P Zyra guide, so it's in perspective of Zyra. However, I hope that it helps you regardless, so I am posting this.

Tell me some additional notes to fix because I am kinda sleepy... It's 1:20 AM in Korea here.

• Alistar: Ignite is a better choice than Exhaust if you solely look at Alistar because if you Exhaust an Alistar, he has Unbreakable Will to counter that. But you have to fully look at the AD Carry because you don't want to Exhaust Alistar at first place. So both are manageable, AS LONG AS YOU USE IT RIGHT.
• Annie: Ignite most of the time. The problem is that Annie's Flash engages are pretty hard to time with Exhaust, so once Annie engages, you would probably have to trade kills, and Ignite is better at that.
• Blitzcrank: Exhaust is more recommended. Against Blitzcrank, because of his Mana Barrier, he will automatically counter Ignite, so when you catch him off-positioned, it's better to slow him down so that he has no chance of escaping to tower and using Rocket Grab. Besides, against Blitzcrank, it's possibly more important to stop his pick potential than anything else, so mitigating damage would be important. However, keep in mind that Exhaust is usually too far away once your AD Carry gets grabbed.
• Fiddlesticks: Exhaust is a bit better. Most will recommend Ignite because of his Bountiful Harvest, but I say otherwise. Fiddlesticks support does not get levels on Bountiful Harvest during laning phase, and his main source of damage is Crowstorm engage. However, Fiddlesticks is relatively weak early game, so if you want kill potential, go for Ignite. Keep in mind, also, that he can Bountiful Harvest your plants.
• Janna: Exhaust or Ignite is both okay, but I prefer Exhaust. You can honestly win with both summoner spells, but the issue is, Janna has too many disengages to kill if the Janna player is good. You win this lane comfortably if you solely look at the support battle, so it really depends on the AD Carry.
• Kayle: Exhaust. Post-6, Kayle owns Ignite, so it's better to mitigate her damage by using Exhaust.
• Leona: She's actually tricky in lower elo :P I do take Exhaust if I want to play the lane safe because I tend to poke in this lane than go for kill, so I would rather disengage than fight post-6 (pre-6, you could actually trade evenly, if not more, by splitting the opponent's AD Carry with Grasping Roots.) However, if you feel like you can abuse Leona during Level 1 (this happens frequently during lower elo - people do not space enough for all the AA pokes.), and seal the deal during Level 2~3, Ignite is not that bad.
• Lulu: Ignite if you look at her solely. Lulu has three movement modifier ( Wild Growth, Glitterlance, and Whimsy) to use during Exhaust duration to mitigate the damage. Also, Ignite counters the heal from Wild Growth. However, Lulu lane is hard to kill (if Lulu player is good) because she has amazing disengages, so if you can't use Ignite to your advantage or if your matchup requires you, take Exhaust.
• Morgana: Exhaust generally. Ignite if you want to cheese it. Now, Morgana's Black Shield counters Zyra's damage pretty hard, but Black Shield cannot counter damage from Ignite, so if you run Ignite, you must look forward to the time that Morgana does not have Black Shield, and go in during that time for a kill to maximize your potential with Ignite. With Exhaust, this advice is similar, except that Exhaust is harder to use in a lane because it gets straight-countered from Black Shield. However, Exhaust is lively during mid-late game, and because Black Shield counters Zyra's damage, I would rather take Exhaust and go for teamfight.
• Nami: Ignite for same reason of Lulu. However, Nami is easy to kill pre-6.
• Nunu & Willump: If a rare Nunu & Willump support comes, kick Willup's butt and show that he belongs in the jungle. Exhaust is more recommended, but Ignite is also good. At this matchup, I would worry primarily about his Blood Boil buff and what it can do at late game more than anything, so I do prefer Exhaust, but again, you can try to dominate this lane early, so more kill potential with Ignite is not bad.
• Sona: Ignite usually. Exhaust is okay in this matchup depending on AD Carry. If you take Ignite, however, make sure that you fully remember the cooldown of Sona's Heal, and make sure you use Ignite before Sona uses her heal.
• Soraka: Ignite. Make sure you use Ignite before her heal, however.
• Taric: It depends on AD Carry Matchup. I can't say more than that about this.
• Thresh: Exhaust and Ignite are both playable, but I do recommend Exhaust against him. Thresh has huge kill potential from Level 2, but he PULLS himself toward you and try to inflict massive damage. Now, you can disengage from that perfectly with well-timed Exhaust. However, Zyra is a skill matchup with Thresh, so if you feel confident, take Ignite. It used to be an easy lane for Zyra. Too bad plants got nerfed in spawn time.
  Zyra: Ignite. Mirror Match is a battle of pride, and you want to win that battle. So screw Exhaust. Remember, you are in a Normal, so don't fret too much :)[/