This is my response to the loss of Innervating Locket. I've experimented with various items and builds to try to find a replacement and finally decided that finding a replacement was a lost cause and the best support is a support that hits hard and drops massive heals with massive AP.

I would recommend this build for those who like to play a more aggressive support role.

The purchase list is the core item list. These are the items you will most likely be taking while playing Sona. Other suggested items are, situational items. Things you should think about getting to adjust to the situation.

First up is Meki Pendantand then either 2x Health Potionsor 1 Health and 1 Mana Potion
When you get a chance to recall, build that pendant into Tear of the Goddessand Boots of Speedif you got enough.

Before you finish your boots of choice (mine is Boots of Mobility) Get your hands on a Philosopher's Stoneto benefit from some regen and the bonus gold for a while.

Finally finish off with Shurelya's Reveriebefore your first Situational item. At this point, you're most likely going to need some AP and if your side is winning or you're getting a reliable amount of assists, then it might be a good idea to get your hands on a Mejai's Soulstealer. Otherwise, build your Archangel's Staffand save that situational slot for later.

Now its time to take a look at how you're doing and what the other team is doing. This is also another opportunity to get that Soulstealer if you're team is suddenly doing much better.
Rod of Agesgoes well with Archangel's if your team is still owning, providing a great boost in AP and some health to keep you alive.
Abyssal Scepterprovides a little less AP than Ages but it gives you Magic Resist if you find yourself getting targeted by AP champions too often.
If you find your opponents to be AD heavy then either Thornmailfor that damage reflection or Frozen Heartthat gives AP through Archangel's and ASPD reduction for whoever is chasing you.

And finally, for your last core item on the off chance that your game lasts that long, is Rabadon's Deathcapto make your AP really shine.

If for some reason you still have a slot open after the Deathcap, then either grab one of those situational items you missed, or a Lichbaneto make your passive right-click really hurt, or whatever else you may feel you would like.

For this build, there are potentially 3 legitimate choices when choosing which boots to use.

Boots of Mobility: The boots are my boots of choice for Sona. They give the standard level 2 movement speed that all boots get plus a passive that raises your movement to level 5 if you haven't damaged or been damaged by anyone for 5 seconds. Healing does not reduce your movement speed with this passive. If you combine Celerity with these boots, you have a movement speed of about 460 from the speed burst and then drops to the 430's when it wears off.
One of the main tactics that I am allowed with these boots is kiting. In a game where the average champion has level 2 movement speed, most have no chance of catching up to you or running away from you. Since healing does not affect the passive, you can safely dart back and forth in front of an ally who is trying to run away, providing them with heals and movement speed boosts. You can also, dash in quick, take a pot shot with Valor and buy some time with Celerity until your 5 seconds are up and you're back at level 5.

Mercury Treads: This is the standard, must have boots for most players. It provides 25 Magic Resist, a killer for any AP build without Magic Penetration, and 35% duration reduction from Crowd Control abilities. I highly recommend these for anyone who finds themselves even slightly frustrated by CC or just because your opponents are AP heavy. Caution though, with these boots, you will find yourself less room for error in casing and escaping situations where movement speed is critical. You may want to put an extra point or two into Celerity earlier.

Boots of Swiftness: An alternative for Boots of Mobility. It grants unconditional level 3 movement speed which puts you a step faster than the average Tank or Caster, and proving a more consistent offensive kiting experience without the loss of speed for dealing damage and an easier time chasing. The only thing missing compared to Boots of Mobility is the rapid movement too and from lanes.

Ionian Boots of Lucidity: A curious new addition to League of Legends that provides 15% cooldown reduction. I have not experimented with these boots and I don't plan on it since I believe it would better suit other more non-supportive casters but, these provide the missing cooldown reduction that would put this build at 35% reduction (5% away from the 40% cap) that other Sona builds pride themselves on. The way I see it, that puts sona at 2 second cooldowns as opposed to 2.5 second cooldowns without it.

For summoner spells I went with Flashand Clarity.

Flash is my favorite escape spell and I think is the most balancing for this build. With boots of mobility, Song of Celerity, and Shurelya's active ability for those emergencies, she already has tons of speed that could rival Ghost. With that same speed she could almost make it to adjacent lanes before Teleport could take her there anyways. On the other hand, with Flash, it gives instant movement and potential wall jumping.

Clarity is also something that works well with my play style. I play a fairly aggressive Sona with her skills so even with all of her mana regen masteries, runes, and items, I still run out of mana especially in early game and having to recall or hug the tower while you wait for it to recharge is a waste of potential minion kills. Mana is not much of an issue in late game but, it could also mean pushing those towers a little longer or keeping up that defence because you were able to restore everyone's mana and keep them in the game.

Skill Sequence is very important early game and can make or break it. The sequence listed here is good for a game where you want to be aggressive but you also need to be able to rely on your healing to keep you or your partner going. There are two other alternative extremes that I make use of when their necessity appears to be obvious.

Pure Offensive:

3x Hymn of Valor, 1x Aria of Perseverance, 1x Song of Celerity by level 5. Then put a point into Perseverance for level 7 and then finish of Valor. This makes Sona a very hard hitter that will turn away most champions when they get hit by it and then right clicked early and mid game even without any AP. I recommend this if you are Soloing or when you can trust your partner (and yourself) enough not to get hurt too much too often.

Supportive:

1x Valor, 3x Perseverance, 1x Song of Celerity by level 5. Then bring Valor up to level 3, and put another into Perseverance if you still feel the need. Otherwise, finish Valor. When your partner is reckless or very aggressive and is dishing out good DPS, you can afford and probably need to put some extra points into healing. Don't put too many points into Perseverance too early though, you and your team will suffer for not having that extra push in damage from you.

Rule of 3:

Sona's skills are relatively weak at levels 1 and 2. Valor can barely kill a minion at less than half health and Perseverance is a waste of mana that can barely heal a bar of health at those levels. Celerity on the other hand, is only useful to give yourself and nearby allies a slight movement boost and can stay at level 1 until the other two are maxed without being missed too much.
So, if you have some other skill build, remember that her skills are good enough when they hit level 3 or higher.

Well, this has been my guide to how I build Sona. Any comments or suggestions to the build would be greatly appreciated including requests for further clarification on Sona.