Note: The build will get updated soon.

Mine or Sona's?
Well, mine: I'm Sproutak and my favorite support champion is Sona. Nothing more is important.

Sona's introduction: She's really great support champion, she hasn't such a good heal like Soraka or Nidalee, but her auras offset low heal. My build is focused on auras, AoE support done by combination of items and skills.

I use marks for magic penetration, seals for health, glyphs for mana regen and quintessences for health. Magic penetration isn's too important for Sona, so marks can be replaced with mana, mana regen or ability power, which is quite useless, good only for Q ability. I use seals and quintessences for health, I don't die so fast at early game, you avoid harass and so on, but if you think you are careful enough you can pick mana regen seals and quintessences for movement speed. Do you think I overestimate mana regen? Belive Sona need it a lot in early game, until she get Tear of the goddess.

Well, items in this build aren't focused on ability power, tanking or whatever, I focus on auras and supportive items. Most important item in this build is Shurelya's Reverie (that golden crown inside the fire). If you get how to use it, you can really help your teammates a lot, during killing fleeing enemies or to escape. I saved once my exhausted teammate using this item and E skill combination, I think it made him faster than without any buffs and debuffs.
Next item are Mercury treads, sure these are the best shoes in the game, except e.g. Ninja tabi for Jax. O.o
Than you need to evolve Meki's pendant into Tear of the Goddess, making you have much more mana during game. I don't have often all 1000 bonus mana till game over, but never mind. As a Sona you don't need so much AP for supporting, so evolving into Archangel's staff can wait.
Next item I'm buying Aegis of the Legion, tanks don't sometimes buy this, but if so, auras are stacked. It hasn't so much bonuses for your allies, but it's quite cheap, and it give more armor, health and magic resist for you. During composing this item, buy 1st Ruby crystal, next it depends on your opponents, check who is most fed, check if you need MR or armor.
The last item before Archengel's staff you need Rylai's Crystal Scepter, making you slow fleeing enemies from the fight by using Q skill.
Just only Archangel's staff and the build is done, often the game ends right before your Rylai's Crystal Scepter is done, so your 6th slot can stand empty. You can buy elixirs for AP, health and Oracle. If you really have a lot of money, buy Soul Shroud.

Maxing your W or Q skill only is useless, because if you have high heal, you can't save your turret alone (e.g. if your teammate die or need to go to shop and you are alone on line) and enemies will destroy it. If you have Q skill maxed, remember level 1 heal is such awful. On level 5 do level 1 song of celerity, if you forgot, do as fast as you can in next levels (7th :) ). Don't underestimate this skill because of it's buff during 1,5 sec after pressing E and it's helpful for having purple power chord, which can work as bad Crystal scepter. :)

Passive is quite good, but you need to check it's status. Blue one is good for pushing turrets in early game and especially for harass. Once i stolen Baron with this power chord. ^.^
Purple one is great for ganks and green one is, I think, very useless, if I use it I'm always like "Oh no wasted power chord..." . It's maybe good for e.g. Nunu's or Katarina's ultimate, but I don't use it a lot.

I use always Clairvoyance and Flash. Flash is simply the best escaping spell, especially through jungle, useful for every champion. If you know how to escape across the walls you often save yourself. This is image showing few places where you can use flash:

You can use flash on many more places, I just hope you get the point.

Clairvoyance isn't difficult spell, just use it every time it's not on cooldown. Remember it see into grass even from outside and it don't reveals invisible items and enemies, even if you got Oracle. O.o

On about 00:15 place the eye between the enemy respawn steps and nexus, to see where are they going and which items they got. If they have a jungler, you can check Ancient golem to see if you can try first blood. :) Again there is a picture you can see where you should place the eye:

Check enemies Blue and Red, to see jungler's status, check bushes between bot line and dragon or top line and baron and check sides of mid lane to prevent ganks. On bot and top lane corners are 2 bushes, if you see enemies disappeared in it, wait a sec and place cv between them, you will see both bushes and enemy near waiting for your Q and powerchord. :)

Pros:

Quite good harass with your Q and powerchords
Good support during all the game
Good mobility to all team
You can easily let your ally survive
AWESOME UTLIMATE, for attacking, fleeing and especially to continue tank's fight start
Very surviable champion in late game
Thousand of auras

Cons:

Die fast in early game
Quite low heal
Problems with mana in early game :-/

Well, I screenshoted my match history:

Yes, if I scrolled down, you saw few defeats, so I didn't ^.^
It wasn't always because of mighty Sona, but If you get when to use the Shurelya's, you can save your teammates a lot of times and enemies don't get fed. ;)