Hi there! I play league very often and my favorite role is support. Many think that support is boring because you're not "allowed" to take kills etc. But with my build that changes. I get ~ 9-10 kills every game but still have ~20 assists per game. Atm, I have a 70% win rate using this build with sona. The best ability that sona has is her Song of Celerity (E) that gives her and allies around her movement speed. And now you may say, "But Orbit, it's her Hymn of Valor (Q) that's the best because it deals damage". Well, it's the second best, why I chose the Song of Celerity as the best is because of its scaling. The more AP you have, the more movement speed. You can escape easier, you can engage easier, you can assist easier, you can kill easier. The build bellow gives you enough AP so you can one shot squishy champs and enough movement speed so you always run in about 600-700 movement speed. If you want me to upload some sona gameplay make sure to leave a comment and a thanks. (my power went off while I was making this thread so I needed to start over, I got a bit mad so that's why I didn't describe why you should build the items so well. Sorry for this.)
The Build
1. You should start off with Spellthief's Edge, 3 hp pots and a trinket
2. Start to build an Ardent Censer, I recommend to start with the Forbidden Idol then the Aether Wisp.
3. When you have your first item you need a lich bane, start with the Sheen then the Aether Wisp and lastly a Blasting Wand
4. Now you need a Luden's Echo, build it however you want.
5. Get yourself a pair of boots, I recommend Mobility boots.
Now you have your core items, you should move in about 600 movement speed and have a good amount of ap already.
6. You can choose what you want to build first, a Rabadon's Deathcap or a Morellonomicon. If you want more dmg then buy rabadons first, if you want more mana and CD buyMorellonomicon first.
7. Now the last item you should buy is a Zhonya's Hourglass. Zhonya's can be a lifesaver and it especially shows when you build it for sona. Sona has quite a low CD on all of her abilities so if you take Zhonya's you can change a whole team fight.
That's it!