There is no reason Sona should use any other Masteries or Runes than the ones shown in this build.
Sorcery and Intelligence in combination with 9 Greater Glyph of Cooldown Reduction are necessary to achieve an initial cool down reduction of 15%. This is later taken into account when building items because only 25% more cool down reduction is needed to reach maximum cool down reduction. These masteries/runes help decrease the cost of the build by reducing the number of cool down reduction items you need to buy.
Because you must choose Sorcery and Intelligence , it is logical to finish the offense side up to Archaic Knowledge to boost your damage and finish the utility side up to Presence of the Master to decrease the cool downs of your summoner spells.
3 Greater Quintessence of Health are necessary to provide Sona enough survivability to survive very early game laning.
9 Greater Mark of Magic Penetration are the only logical choice for marks. They will provide you with another boost to you damage output, especially early game. These combined with Archaic Knowledge will help you harass early.
9 Greater Seal of Replenishment are needed to provide you with enough mana to stay active in your lane early. It also provides a boost to your mana regeneration that you will appreciate throughout the game.
As a support, you must have Clairvoyance. It's uses are just too plentiful for you not to take it. Flash is also necessary to position your ultimate or escape a bad situation. With Blink of an Eye , Mystical Vision , and Presence of the Master these cool downs will be relatively low, which can make all the difference in the world.
Hymn of Valor is the first skill you should get to allow yourself to ready your Power Chord to harass early. Also, the aura will allow your laning partner to harass more effectively as well. In most cases, you won't have a need for Aria of Perseverance until level 2.
At level 3, I choose to level Hymn of Valor over Aria of Perseverance to keep up my ability to harass early. Level 1 Aria of Perseverance is enough to sustain yourself and your teammate.
At level 5, I choose level 1 Song of Celerity because at level 6 you will get Crescendo, meaning if you didn't get Song of Celerity at level 5, you won't be able to get it until level 7. You will want Song of Celerity to move more quickly around the map with mid game fast approaching.
From here on out you will want to max Aria of Perseverance first since Hymn of Valor loses its effectiveness as the game goes on. Then maximize Hymn of Valor, and finally Song of Celerity. Always upgrade Crescendo as soon as possible as well.

Core Item Build (Always buy these items!)
followed by or

After much thought, I have decided Sona needs 4 things, presented in order of importance:

1. Mana Regeneration
2. Cool Down Reduction
3. Survivability
4. Ability Power

Mana Regeneration is most important to Sona for one reason. Once you run out of mana, you must wait a certain amount of time for your mana to regenerate back to 65 in order to cast another spell. Therefore, once you run out of mana, cool down reduction is no longer of any importance because the time it takes to replenish 65 mana is longer than the cooldown of your basic skills. I like looking at mana regeneration as cool down reduction. For instance, at level 1, with Meki Pendant and all the masteries and runes I suggest, it will take 14.93 seconds to replenish 65 mana. This essentially means the cool down of your skills is 14.93 seconds. So, by boosting mana regeneration in the beginning of the game, you can reduce the cool down of your basic skills once you run out of mana. This gives you a stronger lane presence. Once you buy the Chalice of Harmony, your mana regeneration will be high enough that you can continuously cast a skill every 5.96 seconds without worrying about mana. This means you should b back as soon as you have enough money to buy Chalice of Harmony because it cuts down your cool downs dramatically.

Note: If you are able to cast one skill continuously, cast Aria of Perseverance. With the heals and defensive auras it provides, it is highly unlikely you or your lane partner will ever die in lane. Even if you aren't providing damage, your lane partner is free to harass at will because he/she is constantly being healed.

Note: You are squishy early game! This means don't get near the enemy and don't push minions. Think of yourself as a hit and run character. Slowly pick at the opponents health. Since your lane partner should be focused on getting minion kills, you can focus on harassing your opponents. Wear them down by occasionally auto-attacking them and targeting them with Hymn of Valor, and soon you will be able to zone them. A crucial strategy is to gain bush presence. It is fairly easy to gain bush presence by using Clairvoyance on the bush to see if any enemies are hiding in it. Many times they run from the bush once they see you have used Clairvoyance. This leaves it open for you to take. Once you gain bush presence, you can pop out of the bushes to attack them and then pop back into the bushes so they can't attack you. The minions will stop attacking you as well. If they have bush presence, they're the ones who will pop out and kill you quickly because you have no survivability early game. When I say quickly, I mean super quickly. You are very, very squishy, so try and control the lane and stay in the bushes.

Cool Down Reduction is second most important to Sona. The quicker you can cast spells, the higher your healing and damage rates will be. Once, you have acquired Chalice of Harmony, you will have plenty of mana regeneration for awhile. So, you need movement speed and more cool down reduction. Obviously, you must purchase Ionian Boots of Lucidity. At this point, your mana regeneration will barely be able to keep up with your cool down reduction. Thankfully, there is a cheap item that increases mana regeneration and cool down reduction. That item is Fiendish Codex. Once you have acquired this item, you will have maximum cool down reduction and plenty of mana regeneration, thus satisfying the two most important aspects of Sona.

Note: Satisfying these two important aspects of Sona costs a total of 3185g. This is extremely important because Sona is not supposed to farm, so it is difficult for her to obtain gold! A cheap build is key! Those builds you see with Lich Bane are good, but how do they ever afford it? It is much more important to scale well throughout the game. With full cool down reduction and high mana regeneration, you will be able to help your team win early. It should also allow you and your lane partner to win your lane.

The first three items you buy should always be Chalice of Harmony, Ionian Boots of Lucidity, and Fiendish Codex, in that order. With cool down reduction and mana regeneration out of the way, you need to focus on Survivability. You must buy one of two items: Guardian Angel or Zhonya's Hourglass. If you feel like your team is losing and you've been dying a fair amount, or you aren't very skilled with Sona, buy Guardian Angel. If you feel like your team is winning and you've been able to keep yourself alive, or you're quite skilled with Sona, buy Zhonya's Hourglass. The 100 AP will boost most of your skills. The key here is you need a way to protect yourself in a team fight. Zhonya's Hourglass's active is a great way to get someone to stop targeting you. With Guardian Angel, no one will target you in the first place. With one of these two items, you should now have just enough survivability to get by and survive.

Note: If you keep dying after purchasing one of the items above, go ahead and buy both of them. With both, you should have plenty of suvivability.

After satisfying survivability, all that is left to do is make yourself more powerful by focusing on Ability Power. The best way to do this is to buy Rabadon's Deathcap. If you bought Zhonya's Hourglass and feel you aren't in any immediate danger of dying, skip Guardian Angel and go straight to Rabadon's Deathcap. If you feel as if Zhonya's Hourglass didn't give you enough survivability, buy Guardian Angel before buying Rabadon's Deathcap. If you bought Guardian Angel first instead of Zhonya's Hourglass, buy Zhonya's Hourglass next and leave Rabadon's Deathcap for last.

Congratulations! You have now completed the perfect Sona build. Let's look at her final statistics with a full build at level 18.

Mana Regen per 4.2 seconds (When you have 65 mana left): 66.39
Cool Down Reduction: 40% (Basic skills take 4.2 seconds to cast)
Notice our mana regen per 4.2 seconds is higher than the 65 mana you will need to cast a spell, allowing you to continuously cast Aria of Perseverance during a team fight.
Armor: 185.4
Magic Resistance: 98
Note: These numbers don't include the 3 second long aura provided by Aria of Perseverance. With that aura you would have 225.4 Armor and 138 Magic Resistance. That is a lot of survivability for a support.
AP: 365.04 This increases your heal by 91.26, damage from Hymn of Valor by 255.53, and damage from Crescendo by 292.03. Who says AP is bad for Sona? Your heal increases by 76%, and the damage from Hymn of Valor doubles!

Thanks for reading my guide, I hope you find it useful!

I know I don't cover everything you need to know in this guide, but I think I cover enough to show why this build is "perfect".

This is just gonna be a list of random things/stories that are cool/important.

Story:
Recently in a game that was roughly 50 minutes in, my team was hanging out in the middle of the map killing minions. Suddenly, I was pulled through the wall by Blitzcrank into the area where wraiths spawn. All 5 of his team members were there with him. Things looked very bad at this point. I should have died instantly and the opponent team could have won the game then and there. However, once I was pulled through, I activated Zhonya's Hourglass. My team saw I was still alive, so they rushed to help me. Once Zhonya's Hourglass's active ended, they killed me instantly. However, due to Guardian Angel, I was revived. As I was reviving, my team had started fighting the enemy. As soon as I re-spawned, I used Crescendo on 3 enemy champions, allowing us to kill one. I was able to back away from the fight and heal from afar. We ended up losing only 1 team member, while scoring an ace for ourselves. We then pushed and won the game. This is an example of how the Guardian Angel Zhonya's Hourglass combo works to provide plenty of survivability and can turn the tide of a game.

Here is my ideal team:
Jungler: Amumu
Top: Heimerdinger
Middle: Ashe
Bottom: Sona and Miss Fortune

First off, I think Ashe's Enchanted Crystal Arrow is the best initiator in the game. Everyone watches that arrow and naturally wants to attack whatever it hits. It is a clear signal that the battle has started. He is also a great AD carry and can control the movements of the enemy with his slows.
Second, jungling Amumu is my main. I know first hand he can clear the jungle by 3:50, meaning he jungles very quickly and is never in danger of dying. I also know he ganks lanes well before level 6 with his Bandage Toss. At level 6, the Bandage Toss Curse of the Sad Mummy combo is devastating. If you time it right, your opponent will be immobile for 3 seconds. This equals automatic death. He is a very solid tank, and has skills to protect his teammates.
Third, Heimerdinger can easily hold top with his turrets, and during endgame can facilitate a push with the turrets. He is also a capable AP carry.
Fourth, Miss Fortune is a great early game harasser and fantastic end game AD/AP carry. Her Bullet Time is a fantastic AOE ultimate that deals huge amounts of damage.
Finally, there is Sona, who keeps everyone else happy with her auras and heals.

Ideal Situation for Ideal Team:
You are pushing a turret. Heimerdinger is slowly inching his turrets forward while sending in rockets and grenades at the enemies around the turret. Ashe sends Volley after Volley into your opponents, while Miss Fortune lets off the occasional Make it Rain. (Let me point out that if the enemy takes the fight to you, Heimerdinger will use UPGRADE!!!, thus separating the enemy team from each other, and the fight is as good as won. No enemy will want to pass the wall of turrets) Any damage your team incurs is easily remedied by your heals. Suddenly, Ashe decides enough is enough is enough and shoots an enemy squishy right in the face with an Enchanted Crystal Arrow. Amumu, seeing the arrow, instantly jumps to the squishy via Bandage Toss and catches the enemy within Curse of the Sad Mummy. Right before Curse of the Sad Mummy ends, Sona uses Crescendo on the enemy champions to continue the stun/snare. Meanwhile, Miss Fortune uses Bullet Time on all the enemies stunned by Amumu and Sona, dealing massive amounts of damage, Ashe and Heimerdinger destroy the other team with large amounts of DPS, and Sona constantly heals everyone and deals modest damage. The fight should end quickly in your favor. This is the best strategy I can come up with in League of Legends, and I absolutely love it!

Sample Game:Notes:
1. Caitlyn was my laning partner. She had 192 minion kills, while I had 9. This is exactly how it should be. By letting Caitlyn get all the minion kills, it allowed her to buy awesome items to dominate the other team, as shown by her awesome score of 12/1/5 and farm of 12.5k.
2. I had zero kills with Sona, but 24 assists. This is also how it should be. Let your teammates get kills because they need the money, not you.
3. Note that the game only lasted 29 minutes, but I already have my core items ( Chalice of Harmony, Ionian Boots of Lucidity, Fiendish Codex), and all the pieces to Guardian Angel. Sona is unable to accumulate gold like other champs. This is why a cheap build is key.