Sona is the best support ever. She can do everything in adventage to all the team. Healing, buffing as no one else, chase/escape, an amazing stun and an almost unknown passive. This champion isn't a great source of damage and it's farming has some troubles in early/mid/late games. So she must assist everyone to get some gold. Always stay with someone or you are useless. Summoner's Rift master support may buy oracles whenever she cans, even when there's no stealth to prevent wards when killing Dragon or getting Baron.

In the whole game, mana is the main cause of disaster when playing Sona. So I'll take mana per 5/lvl Greater Quintessence of Clarity and Greater Glyph of Clarity , this plus Soul Shroud helps when getting dry often.

Cooldown reduction helps anyway, it would make yur abilities more frecuent, that fraction of second could be the differences between a kill or a death. Also, her passive gets better with that frecuency. As Greater Mark of Focus and Greater Seal of Focus gives a flat buffs, it will help in all kind of games.

Magic pen... yeah. You're not a nuker, or something similar. Don't take it.
Armor or MR... maybe. You have a lot of chances to survive only with abilities and this build. I mean if you have already used all oyur abilities and spell, runes won't be the difference.
Mana... not the solution. It doesn't matter how much mana you have if you're oom, nothing to do here.
Health or HR... kidding? You heal like a *****!

Going 9/0/21 is the best for any support.

I know that choosing between Expanded Mind or Awareness is polemic. However, I assure that leveling up isn't as important as mana when playing Sona, because No Mana=Death!

Staying near the team is very important so that you can ***it them, if you die Good Hands is the best alternative to quickly arrive next to your team.

You may think lol this build sucks or wtf, anyway this is what I recomend, it's a thing of numbers. While we become buffers for each teammate Sona has around these buffs multiplies. It's even funny to see the buff line filled when Sona is around.

If we start a team fight and everyone is there this build will give to may team a total of (not including Sona's passives):

+50% Cool Down Reduction
+60 Mana Regeneration per 5 secs.

+150 AP
+225% Spell Vamp

+60 Armor
+75 Magic Resist
+40 AD

+100% Life Steal
+100% Attack Speed
+150 HP Regeneration per 5 secs.

It's 6 vs.5!

As you're playing like a support the main idea is to rank your Heal without forgetting the other abilities, because Hymn of Valor is very usefull when getting turrents or jungling or Song of Celerity is the best way to escape or chase.