Welcome to my Sona build. I've used Sona for over 500 games now, have tried many, many builds for her and I find that this is the best build for my playstyle. Let me begin by saying that this is NOT a support build for Sona. I do not believe that Sona is a 100% support character. Go ahead and laugh, but let's look at the facts:

Sona's passive is offensive (and amazing).
Sona's Q (Hymn of Valor) is offensive.
Sona's R (Crescendo) is offensive.
Sona's defensive/support abilities are her W (Aria of Perseverance) and her E (Song of Celerity). That's it. That's 3 offensive abilities and 2 defensive.

Don't get me wrong, Sona is obviously a support champion, however, her damage potential is incredible and this build simply capitalizes on that.

So on that note, let's begin the guide!

Pros:
Incredible damage coupled with amazing range.
One of the best Ultimates in the game in my opinion, especially for a support character.
Amazing healing ability with a low cooldown.
Gains gold very easily, even if she's not getting kills, with assists and farming alone.

Cons:
This build will leave you very vulnerable, so positioning and map awareness is crucial to your survival.
Song of Celerity is not a very good escape mechanism, so don't expect it to save your life.

3 X Greater Quintessence of Potency - Gives you that extra AP you need. Combined with your Hymn of Valor aura and a Doran's Ring, you'll find yourself getting First Blood more often than you'd think.
9 X Greater Mark of Insight - Magic penetration is always handy.
9 X Greater Seal of Replenishment - Mana regen, which combined with Clarity, allows you to really spam your abilities.
9 X Greater Glyph of Focus - CDR, Crescendo comes up more often, 'nuff said.

I go with the standard 9/0/21 build for Sona, with a point in Clarity and Ghost. I find that Clarity is essential for this playstyle, because you will be spamming your abilities and until you can recall to get that Tear of the Goddess, you will find yourself having mana issues early on. Clarity, for me, always seems to be popping back up at exactly the right time so I can replenish my mana and continue harrassing.

I level up Hymn of Valor first, followed by Aria of Perseverance, and finally Song of Celerity, leveling up Crescendo when it is available (obviously). I normally put one point in Song of Celerity by Level 5 and level it up last.

Ghost and Clarity are the two summoner spells I find more beneficial for this playstyle, but you can choose whichever ones you want of course.

I'm not done with this guide and will be adding more later, but feel free to leave any comments and opinions! Thanks for reading.

Here are some games I've had with this build. As you can see I sometimes substitute my sorcerer's shoes for boots of lucidity, and sometimes I get a lich bane, it all just depends on how the game is going.