Sona's best keystone is Stormriders Surge

Just think about it for a second

She can't proc TLD so well because the game thinks her 4 sources of damage coming from her poke are 2, she can auto reset her passive to get if off but that often leaves a bigger window open to take enemy harass

Windspeakers is okay if you plan on maxing your W and being a healbot most of the game, but ehhh...

But Sona likes safety and she pokes hard and after she has spent her poke she doesn't need to be near the enemies until it's up again, this keystone has saved my ass so many times and it's hilarious to be caught by a slow from an enemy support and just zoom out to safety only to do it again 10 second later, she can pretty much always proc it in botlane early on a squishy champion like most adc's with a Q passive combo and lategame when you have your artifical scaling items like Lichbane and Ludens you can start reliably dealing the required 30% damage sometimes even to tanks

I always see people asking me if I am trolling or have the wrong masteries when I take SS on Sona, then I explain it to them and it all makes sense (to most)