Summoner spells: Flash for obvious reasons, and the other is dependent on your carry. For total unknown the best is Exhaust. It can still DESTROY onehit combo enemy champions for team fight. If your teammates agrees, then Heal is better, but that means he should get exhaust. As Soraka ignite is useless, because you don't really good at killing, or CC the enemy. Especially mobile enemies. If your carry is known skilled, then Teleport is a good decision. It will force you to avoid real confrontation, but it helps a lot, because you gona recall often through the whole game.

Ability order: Simple: Heal, Starcall, Silence aura, then level in this order: Ultimate, Heal, Starcall, silence aura.
Reasons:
I found the Starcall really risky against opponents with brain. Especially true against "all in" builds. Better to be safe behind, and for that heal is the best to level. The health cost of heal is always 10% max. health, but with more AP, and higher ranks one heal heals more. More heal means, that you don't need to cast it that many times, and that means you don't loose that much health. The silence aura's damage is meaningless, and it rarely roots enemies who has brains. So leveling it is pointless, but the silence part can really save your teammates if well timed, and placed.

Item order, and reasons: After starting items upgrade gold item to next level (500 gold) as soon as possible for more gold, and health from minions. Then depending on whether you win or loose the lane. If you push hard, and win, then get Ward item, then boots so you can see jungler in time, and run away. You have very little escape ability as Soraka. If not, then buy CDR part of Zhonya, then boots. That early AP, and CDR will mean a LOT during fights, and healing the carry.

Next item should be Zhonya. Smart enemies WILL focus you. Dumb enemies are rather stand, and wait next to your Zhonya, than damaging others, or trying to survive until it wears off. Even if they don't focus you it gives a lot of AP, and that gives faster healing to teammates. Next is Talisman, because movement speed is damn useful. Good for engage, good for disengage, good for running away from lost team fight, good when someone focuses on you hard, good for catching up faster teammates. Combined items only good if enemy isn't mobile. Kinda rare these days. Unless your enemy is pure AD you need that Mikael or you get sniped. The increased heal also helps. If enemy really goes for onehitkill AP, then you might need it before Talisman. Last item(s) is Rabaddon for tons of AP, or if you use Eye of the Oasis, then Seraph first, and Rabaddon second.

Tactic:
Stay behind, heal your team, and spam Starcall on Enemy. Focus on the best dps first. The best DPS is not always the ADC! Your silence aura will have great CD so use it when enemy assassin goes on your carry, or you. Assassin need their skills to destroy their target, and that is what you deny with the silence.

Pros:
Good survival compared to other "glass cannon" Soraka builds.
Good mostly scaled healing through the game.
Gets CDR very quickly.

Cons:
People often forget Zhonya.
Need to recall often, because runs out of health fast.

Problems with most other kinds of build i tried:
Warmong, Spirit Visage, Tons of health regen, and health. The main problem is, that before you reach that 3k. health this kind of build weakens you. Your healing costs more and more health without healing more on your team. By the time you get that super health regen OUT OF COMBAT! (So if you get focused, then you must not heal others except ult to survive, or die just as fast.) The game is usually decided. Unless someone does some serious mistake you already won or lost. It's around 35-40 min.. Might be different on very high ranks, but you have to get there first. Next patch will change Warmong so this build might become more reliable.

Pure AP:
Mine is almost like that, but negating Zhonya is CRAZY! A single assassin, or fighter WILL tear you to pieces without it. Enemy must be really dumb not to focus you, but if enemy is dumb, then you win unless your team is dumber. Runes are always for early game, and in early game you need survival. Soraka heal has great CD during early game, and has low range during whole game (except ult of course). Smart enemies will try to damage you as hard as they can during early game, but if you are a damn offtank, then they won't gain that much from it.