Sona breaks the idea of cooldown reduction with her new ultimate passive in patch 6.14.
She’s been in and out of PBE with her numbers being fiddled with every day to adjust her to the meta. While buffs for the Maven of the Strings were necessary, Riot didn’t want to completely break her. Even though they were careful with tuning her, she still got pushed to live broken.
Sona’s Crescendo originally gave bonus stats to her other abilities, but it was recently changed to give increased cooldown reduction to her base skills. This means she can have 67 percent cooldown reduction with the right items and masteries at max level.
Her Hymn of Valor (Q) was nerfed a bit, making her poke less devastating, but her Aria of Perseverance (W) was buffed, so she gives more shields and more health. Song of Celerity (E) was also buffed, so the movespeed doesn’t decay anymore and it grants more. Her passive, Power Chord, also hits harder and shrinks the target.
She currently sports a 58.48 percent win rate according to Champion.gg, which is roughly five percent higher than Soraka, who holds the spot at number two for win rate.
To combat Sona, pick a high damage poke-based support, like Brand, Karma or Zyra. If you do crazy damage to her, she won’t be able to keep up with the healing and she’ll probably end up draining her mana. Stay awake from mana-hungry short-range supports like Tahm Kench and Alistair. As an AD-carry, lock her down with Jhin or Ashe and burst her while her cooldowns are still low.
Ashe goes phenomenally with Sona since you can slow and stun your target forever - they won’t be able to get away from you. Double your engage and speed with Sivir for a good end-game team composition.