I tend to lean towards
as I see Heimerdinger's main role is of one of utility. Being able to assist your team with teleport should they be overextended and a gank occurs can turn around the lane and in turn win the game. Flash can be used to reposition yourself within team fights and escape ganks in laning phase.
SEE BELOW FOR INDIVIDUAL EXPLANATION
Teleports your Champion to target nearby location under your mouse cursor.
Flash provides all around benefits to Heimer's playstyle. The summoner spell can reposition Heimer should he be out of position (caught by a gank) or whether you are positioning for turrets and a stun. Though with such a useful ability comes the long cooldown. Without the mastery the cooldown is 300 seconds. That provides a 6 minute window, when you are not safe. Junglers and laners can time your flash cooldown in order to know when you are most susceptible to a gank.
In laning phase flash should be used in a purely defensive manner unless you feel confident in landing a concussion grenade using flash (for a jungler gank). Though this is not recommended as Heimerdinger has little kill potential and it may lead to your jungler wasting his time. When the opponent ganks, you want to hold flash for as long as safely possible. Blowing flash at the first sight of the jungler is not the best thing to do as that instantly opens up a window in which they can gank again. Try landing a concussion grenade first and if all else fails, flash before their crowd control gets you.
In teamfights flash is used in order to reposition. If you get caught too far from your team you can flash to get back into position. Flash can even be used to set up for your turrets. As during teamfights your turrets provide an AOE slow, you can flash to position yourself next to the enemy team (once their cooldowns are burnt) and slow them with turrets and concussion grenades.
After 4 seconds, teleports your champion to target friendly minion or turret. 3 minute cooldown if cancelled.
Teleport is a spell which allows Heimer to have global presence. Although initially I would have only recommended with against champions like Shen and Twisted Fate, it has evolved to become a key part of my Heimer kit. Teleport can be used to quickly counter gank an opposing lane. Split push in order to set up for dragons and assist your team following a quick push. Though with teleport you have to take care that when you do teleport you are not wasting the cooldown and putting yourself out of position. Make sure that when you do teleport you can't be crowd controlled directly upon arrival as it can quickly lead to death.
In the early game teleport should only be used to return to lane if you die and missing the wave that is coming will leave you severely underleveled. Teleport can be used to counter gank bottom and mid and assist your jungler once you hit 6 as you have insanely high crowd control.
In team fights you can split push a lane and teleport near your ADC when the fight actually starts allowing you to pressure around the map as well as provide assistance in teamfights.
Targets a single champion dealing 50 + (lvl x 20) true damage over 5 seconds. Also reduces the target’s healing and regeneration by 50%.
Ignite, although viable, is a fairly aggressive summoner spell. It allows you to finish off opponents and reduce the healing of enemy champions. I would not recommend this summoner spell for Heimerdinger as it suites a more aggressive playstyle (unless you can adapt Heimer to an aggressive playstyle). It provides more damage but what Heimerdinger really needs is the abilities which can help him survive and assist his team. Ultimately I choose not to take ignite as it forces an aggressive playstyle, which I do not feel best benefits Heimer. Though, it is up to preference as it can be used effectively in mid should you adpot an aggressive playstyle.
Ignite should be used either early in the laning phase in order to put the opponent at a disadvantage, or to finish off / damage an enemy when trading. Blowing ignite early just to lower the enemies hp should only be done when you have the ignite mastery as you gain a bonus 5 ad and ap which although is minimal, adds up.
Exhausts target enemy champion, reducing their movement speed and damage dealt by 30% for 2.5 seconds and reducing their attack speed by 50% for the duration.
Exhaust, is a very strong pick as it goes well with how I play Heimerdinger. Being able to furthur control those who would dive on to you and your ADC and reduce the damage dealt to your teammates by a member can greatly change team fights. Being able to reduce an opponents burst, in addition to reducing attack speed can negate a champions usefulness for the duration.
Exhaust can be used as a strong bait in laning phase as the slow can make it easy to land a concussion grenade as well as your turrets dealing major damage early. If the opposing laner dives on to you, you can fall back to turrets, exhaust -> stun and you have the opponent crowd controlled long enough to kill them through turret damage. When being ganked by an opponent you want to minimize the damage of their highest damage dealer. Exhausting the one who has the highest damage potential can save your life or bait so that you can kill one of them should they dive.
In team fights exhaust is extremely versitile. You want to try to exhaust their AD carry, any burst champion (pre-burst), or a bruiser diving on to your ADC. Using exhaust in conjunction with your ulti slow and concussion grenade can make it impossible for the enemy to kill your AD carry.
Shields your champion for 95 + (20 × level) for 2 seconds.
Barrier allows Heimerdinger to survive longer, and bait the enemy by seemingly having low hp, only to barrier. Being able to survive longer allows Heimer's turrets to survive longer, which leads to more crowd control, more damage, and increased usefulness in team fights.
In laning phase barrier can be used to survive jungler ganks and (just like exahust) bait the enemy into your turrets. Though barrier provides no team usefulessness in team fights other than keeping yourself alive (which that in itself helps the team). In team fights it can be used to keep yourself alive and in turn do more damage.
Your champion passes through any unit and moves 27% faster for 10 seconds.
Ghost provides increased movement speed and the ability to move through minions. The increased movement speed is good for escaping ganks, and chasing in order to get in range for concussion grenade, though the disadvantage of this over flash is that hard crowd control (suppress, stun, knock up) still fully applies, and you cannot flash instantly to make sure you are out of range. Though with ghost you can kite around your turrets and around minions as you phase through them while the opponent gets creep blocked.
Ghost can be used to escape ganks early game (not against hard cc), as you can outrun the opponents with increased movement speed, or by going through minions. In the laning phase you can use it to kite around your turrets should the opposing laner dive on to you (while your turrets shred them). In team fights ghost can be used to position yourself similarly to flash though not as instant (but longer lasting). Using the increased mobility to place turrets and position for rockets is what makes Ghost a viable pick on Heimer.
Removes all disables and summoner spell debuffs affecting your champion and lowers the duration of incoming disables by 65% for 3 seconds.
Cleanse allows you to remove most crowd control's except suppression, knock ups, knock backs, grabs, and flings. In the laning phase it can remove an opponents crowd control and the damage over time from ignite. Although the ability to cancel out crowd control is a great asset in lane, its long cooldown makes other spells such as teleport which can change other lanes and exhaust late game are more effective.
Cleanse can be used to remove crowd control during a gank. You want to try to cleanse the crowd control effects the second they hit you in order to maximize effectivness. Though be careful if you are being ganked by a jungler and a laner that both have crowd control you might want to save the cleanse for the stronger effect in order to survive. This combined with flash or ghost can make any gank survivable.
Reveals an area of the map for 5 seconds.
Clairvoyance, although giving vision and additional map control. Does not provide anything for Heimerdinger. For the most part you can use your grenade and your turrets to keep control of brushes and fog of war. Other summoner spells can be lane changing and eventually game changing, while vision can be provided through other means.
Restores 40% your champion's maximum mana. Also restores nearby Allies Mana by 40%.
Clarity does make up for Heimer's mana problems, and should only be taken if you cannot manage Heimer's mana. Once you begin building tear of the goddess or chalice you have mana regeneration and mana you can cope with Heimerdinger's high mana costs.
Restores 75 + (lvl x 15) Health to your champion and 100% of that to nearby allies. All targets healed will have a 50% reduction in heal amount from other summoner heal spells for 35 secs.
Heal can act similarly to barrier but does not give as much as a shield. In a solo-lane it is much stronger to take barrier in order to bait and survive ganks. Heal can be used when playing support-dinger though I much prefer exhaust for additional peel for the carries.
Instantly revives your champion at your team's Summoner Platform and grants 125% movement speed increase which decays over 12 seconds.
Revive is a spell usually chosen when planning on dieing. Though it provides a large advantage in team fights as it can make it a 6v5 when combined with a way of getting back into the fight. But its large cooldown, and requirement that you must first die makes it ineffective as once Heimer dies, his damage and crowdcontrol from his turrets are gone and still require the cooldown to refresh.
Deals 460 + (lvl x 30) true damage to a jungle monster, enemy minions, or pets.
Smite is ultimately a junglers spell. Should you choose to go Heimer Jungle you should focus on using your turrets for tanking. Smite can be used to secure objectives waiting until it is low hp, and comboing your rockets / grenade with smite, in order to secure the buff. I choose not to take smite as I focus on lane Heimer. (JUNGLE HEIMER NEXT?)