
Ap is very good on Sona. But I usually don't prioritize it over utility/support items like sightstone, Crucible, or locket of the iron solari. After I've got those items though, I will usually build an ap item (depending on the circumstance) Athenes, Morellonomicon, Deathcap, Voidstaff, zhonyas depending on the enemy team comp.
My standard build for Sona is

- If the enemy team has an AD comp, don't buy Locket. Instead buy frozen heart.
- if ahead and getting frequent barons/dragons occasionally I get Banner of Command instead of Locket for super sieging.
- If the enemy team doesn't have a lot of cc, I might buy athenes instead of crucible.
- if you have a team with lots of on hit damage or who love attack speed (teemo, udyr, master yi, azir) you might want an Ardent Censor
- only upgrade to ruby sightstone if you have a complete build with excess gold.
- If it's one of those 50+ minute matches, I sometimes sell Frostqueen's claim and get a better ap or defensive item. (banshees, GA, zhonyas ect)
If we're talking aram, don't build Sona like a support at all. Build her like a glass cannon mid, I'm talking -

You will one-shot squishy adc's :D

There is only one way to deal with a Soraka, and that is to focus her in the beginning of teamfights. Most higher elo will know this, therefor I prioritize defensive items over Ap on her.
My standard Soraka build is

Warmogs is key here. I know, stacking health on Soraka is usually counter productive, and exactly what they tell you *not *to do. The thing is, since warmogs has been changed, it's passive is too good to pass up on Soraka.
when out of combat (not giving or recieving damage) warmogs will give you 3% of your max health as health regen every 5 seconds. This means, with proper masteries and hp regen runes you can easily have 100 health (just tested, at level 18 out of combat I had 180 health regen per 5) regen per 5 seconds when out of combat. This means that you can sit in the backline, healing your team and still receive the passive (healing isn't considered in combat).
The purpose of this is so you can spam heal your teammates without having to rely Solely on Starcall for self sustain. This also means that if an assassin tries to get to you, they can't just one-shot you.
It's actually pretty broken on Soraka, and I won't be surprised if she or it get's nerfed.
Don't build
on Soraka.
is way better, it's active provides some nice disengage or chasing potential for your team, while also providing you with much needed health regen. Soraka is pretty much the only support that talisman is really good on.
Ap is *okay *on Soraka, but in general her Ap ratios are around 40% - 60% which isn't very good. The exception is Wish, her ultimate. If an ally is below 40% health, wish deals double the heal, with an 80% ap ratio which is very good, but I just don't think it's enough to justify an ap build.