I am an agressive Soraka player, the build(s) I do might feel strange for you but it works for me (300 games this season with a 60%+ win ratio).
Thanks for reading!

Depending on how bad/good your adc is you max Q OR W accordingly. If your adc is horrible then I usually max Q; if they are decent I max W and Q alternately; if they are awesome then I max W only and second max Q.

In ranked you have to get used to the fact that EVERYONE on the enemy team hates you and wants to kill you. Now if you feed off their hate you get powerful and you will wreck everyone. Make sure to spam your laugh every time you stay alive with 10 hp :^)

In teamfights, make sure you stay back and not get hit by for example Mumu ult, Shyvana ult, Gnar ult, etc.
The more people you get in your Q and E combo the better.

If Lee uses Q on your carry and you see him dash, use your fast fingers and put the silence below the person who got hit(it's won me so many games against Lees, Rivens and Yasuos)

After 6 Soraka is a global healer and everyone loves her just for that, don't they? Make sure you look at the map and be up to date with the scores and farms of your teammates so you'd know who to ult for and who not to waste it on.

I am in no way whatsoever the best Soraka in the world but the builds I do work for me and in terms of runes.. Well these arent actually bad, I like the poke I get from my AP.
I will start buying runes soon and make a page especially for Soraka.