Hi, I'm Alphaomegalogs, maining support since S10. I don't have a fancy rank (I don't have one at all because I don't play ranked) but I do have a lot of experience playing Soraka. I've tested a lot of builds and this one is my favorite due to its scaling potential and super high heals in the late game.

Pros:

Super Good Heal Power
Amazing Poke
Poweful AOE silence
Quite Fast Arriving at Teamfights
Her newer skins are really good
More healing, scaling
Even more healing

Cons:

Squishy
Mana hungry early game
Low damage
In order for this built to work, you have to be very good at positioning and map awareness

In the early game it is important to poke as much as possible. Try to predict where the enemy will be when your Q hits. Use when your ally gets below 40%, and preferably after you hit Q for bonus healing. Use E if you see an enemy channeling an ability, or if an assasin comes. It can save you from a burst mage too. Use E to poke only if you have a lot of mana and know that you won't be getting ganked before the cooldown is over. Never use it to poke if you took Relic Sheild Relic Shield or your support quest is finished.

Once you have Redemption, you can give your team a massive heal even while you are dead. Use Q to test bushes. If it hits someone, you will get the heal and speed. During a teamfight, if you see that many enemies are grouped, use E and you will have a high chance of somone getting detonated and rooted. You can also use it to control enemy movement by placing it so that they run away from you, or towards a team memeber. Make sure to keep an eye on your teamates portraits. If more than one gets below 40%, press R and you may save a teammate or even a teamfight. It can also turn the tides of scuttle crab skirmishes, top/ mid 2v2s, etc.

At full build, it all comes down to staying alive. DON'T DIE. Stay near (but behind) the team tank or damage dealer. You are very squishy. If the ally protecting you is about to die, and you can't save them, Q an enemy and run. Flash if you need to. In the late game it is very important that you keep your adc alive as they will most likely be the main damage dealer. If you place an E in the right spot, it can guarentee an escape because they will walk onto it right as it detonates. Hopefully at this point you have a lot of dark seal/ mejai stacks, so each Q into W will do 500 HP of healing every few seconds. Plus R, which will do around 700 with moonstone and on low HP allies.