In my opinion Janna is the best support. It has been proven (personally) that the most important thing that make a champion the best support is the aoe CC/auras (especially after the patch that all healers got nerfed at heal). More CC/auras a champion have, more supportive it is from the early game till late. This built, as i,ll explain below, is simply the best. It is not what will give you kills(no need actually), but it is what will save you and your teamates from deaths, leading you to the victory.

is the best summoner spell for a support. With its frequent use you can help your team avoid gangs as well as control some strategic map points (drake etc.) It helps,also, jungler to make counter jungling that give him an early game boost comparing with enemy jungler. As a support you must have map awareness and check some specific points all the time. Get used to it... xD

is by far the most lame (yes, really boring to see in every game 10/10 flash... i want to play league of legends, not league of flash xD) summoner spell, because it gives the opportunity to everyone (having or not blink ability) to avoid a gang and/or gang from nowhere. (that's why you must have it with a support because you ll be targeted much times, especially at early game that you are more viable. Also combining it with Janna's ulti can also be very useful.

Anyone, who has read the introduction, maybe understood that my philosophy concerning support, is "if i live i can support, dead i can only watch". Based on this, i created the rune page.
As far as marks concern, many summoners may wonder "why armor? marks are basically offensive runes". The answer on this is that as support you are not intrested in doing damage. You care about the other 4 teamates that you have to keep alive in order to make much more massive dmg than you, either you have 9 runes magic pen either ap. So, the next step is to think what we can put into marks. Comparing all stats, it's obvius that armor and magic resist are very good, not like seals, but good enough. Between armor and magic resist, i chose to put Greater Mark of Armor because in early game every support goes to bot with the ad carry, like the enemy team.
Thus total armor combining Greater Mark of Armor + Greater Seal of Armor is more than enough to withstand in lane,even without heal.(I'll explain at pros/cons chapter how it is possible).
About glyph, the ideal for every support is Greater Glyph of Cooldown Reduction (i also tested greater glyph of replenishment/ Greater Glyph of Scaling Mana Regeneration but they are useless comparing cd reduction).
Finally, Greater Quintessence of Gold help you early game, because without any creep farm, u can easilly+ FAST
create philosopher's stone + Heart of Gold.

• Greed +1 gold, combining with runes, +4 gold from the beggining (80% gold item till the end of the game)
• Awareness for more xp.
• Perseverence + Meditation + Strength of Spirit for mana/health regen. (useful for early game survavability)
• Quickness for more speed (combining with 2 janna's passives is simply OP.)
• Intelligence for cd reduction.
• Blink of an Eye + Mystical Vision + Presence of the Master to boost summoner spells.

ps. it is useless to lvl up Good Hands because you will die insignificant times. ;)

It's all ok with runes,masteries,summoner spells so let the game begin...
Below i'll write a more detailed item sequence:

_+3_

sight ward

_+2_

~(back at 700g)~~>

_______

philosopher's stone

__+1_

sight ward

_______

~(back at 900g)~~>

---

_+__

_+1_

sight ward

~(back at 1300g)~>

___

_+_

__+1_

sight ward

___

~(back at 1000g)~>

---

____

_+2_

sight ward

____

~(back at 1650g)~>

_+_

_+_

_+3_

sight ward

~(back at 1200g)~>

---

____

_+2_

sight ward

This is the basic stracture for the first 3 basic items that you ll need. Of course this order is ideal and indicative. You can modify it depending your kills/deats/assists/gangs/ oblige def etc.After these items you have to check where you have to focus first, depending on the enemy team.

Usually next steps are: Randuin's Omen ~~> Banshee's Veil ~~> Rylai's Crystal Scepter

PS1 Dont forget to use your active items... ;)
PS2 You can create Frozen Heart instead of Randuin's Omen if your tank want to have randuin's. Also is a good idea to have
randuin's both, cause you can use them one after the other during the team fight.
PS3 With that built+active use of items+good use of spells, Janna is almost immune to death... :P

The last thing that we have to talk about is the skill sequence. As you can see above Janna have to start with Eye of the Storm for 2 basic reasons:

1. 80 absorb dmg at lvl 1
2. 14 atk dmg

Second skill is Zephyr. With lvl 1 Zephyr the benefit is double. Janna gains 8% extra movement speed passive and she can slow an enemy for 4 seconds(!) with 24% slow. If we calculate the 3% passive speed that Janna + ad carry will have, they have an obvius presedence. so if Janna initiate with slow (dont forget your early game "tanky" runes) there are lots of chances to take an early kill.
Third skill is Howling Gale. When u reach lvl 3 you have 2/3 CC available. Knock air + slow. That means that it is extremely difficult to die from a gang and you have more chances to take a kill or harass.
At 4-5 lvl you lvl up your Eye of the Storm for more early game absorb+dmg for the carry.
General philosophy is to firstly max lvl up your shield for max dmg+absorb, then max lvl up your Zephyr because of passive+extra slow and lastly lvl up your Howling Gale.

The advantage of Janna in relation to any other support is that she has too many aoe CC and with great range, resulting in breaking the cohesion of opponents or saving her teamates. In addition she has a very good in duration and percentage slow as well as a shield that early game is OP boost to the ad carry.
However, as every champion, the only disadvantage (compared with other supports) is that she has no spam heal. This is a problem for her and her teamates because she is the only support with that feature. The solution to this is to use the shield when is really needed in order to efficiently and effectively absorb dmg.
Also, concerning that built, the only disadvantage is that except from philosopher's stone (and late game Banshee's Veil) you dont have another item for mana regen or max mana (for ex Tear of the Goddess) not either some runes, so you have to use your skills only when it is necessary. Unless your team, give you blue buff... ;)