Sona - the newest "no weakness" champ
Sona is currently the best support, hands down, by a statistical measuring. The website Champion.gg aggregates data for plat+ games, and shows Sona having a 58% (!!!) win rate, with the next highest support win rate being Soraka with 53%. In other words, Sona is a full 5 points above every other support in the game. This isn't just a high-elo problem either. The website BestBans.com recommends statistically optimal bans for bronze through diamond, and every tier shows Sona with at least a 55% win rate, even though her current pick rate is "only" about 15%, and thus recommends banning her.
If you look on the Champion.gg page for Sona, and look at Champions Who Counter Sona, the top two are Blitzcrank and Soraka, both of whom only beat Sona 47% of the time. That's right - her best counters are still losing matchups.
The issue here is that Sona has no weaknesses. Her defining characteristics are her fight-winning ultimate and her ability to dish out damage and heals during team fights. She is a strong mid game champ who can single handedly deliver a decisive teamfight victory to her teammates. The usual trade-off for a high-octane midgame powerhouse is a weak laning phase and a late-game falloff. Sona has the falloff to some degree (a graph of win rate versus game time shows that her chance of victory decreases as you pass the 30 minute mark), but has an incredibly strong laning phase.
Sona's poke damage at level 1 is second only to dedicated mage champs, and is still close to their level. When she picks up her heal at level 2, only Soraka can keep up with her sustain. When you combine these two abilities, and consider that most players just push these two while ignoring the movement speed altogether until level 9 when it's useful in getting around the map, you have a champ who is nearly unbeatable in lane. Sona doesn't have to expose herself to much counter poke in order to get off the Q and auto-attack that define her trades, so she generally wins or comes out even when she pokes. After that, she uses a single heal and is back to near full, mitigating the entirety of enemy aggression and thus winning the trade.
We've seen this problem many times, where a mid-game powerhouse has too much early game strength, or when a late game hypercarry has too easy of a time in lane, and that champ becomes the de facto best in class. Sona is just the latest.
My proposed solution is to lower her early game impact by reducing the base damage and heal that she has access to at early levels. This would give opponents an opportunity to fight back and win lane, thus having the edge they need to succeed mid game against Sona's incredible team support mechanics.

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