Individually for her abilities:
Her [Q] is undodgeable and won't care if you burn flash or dashes trying to not get hit by it.
Her [W] makes it difficult to trade and out-poke her.
Her [E] gives a fair amount of MS and that can be used any number of ways.
Her [R] is almost instant and thus practically undodgeable, capable of stunning a whole team.
Her passive's utility is strong, especially since it will be up so often.
She becomes stronger and stronger as the progresses, so her early phase barely compares to her power in the late game.
Her auras scale with AP and CDR, making them even more effective in team situations, and there is no limit to how many benefit.
Let's move on to her item synergy:
Ardent Censer lets her [W] proc the attack speed for the entire team, on top of the initial heal.
Lich Bane makes her [Q] pack a wallop, and will make building her as an APC hurt.
Aegis of the Legion makes her [W] more efficient.
Due to her playstyle, her only counters are to pick the hard-engage tank supports Thresh, Blitz, Leona, or go with Braum and block all of her damage. Annie can maybe punish her, too, but that's more of a skill-matchup.
Trying to counter her alone doesn't really help since her greatest strength is working with her team, and a smart Sona does not stray from her team for too long, so picking her off is difficult without wasting everything only to have to deal with her carries right afterwards.
When directly comparing her to another support, they likely have to land skillshots or manage a more convoluted method of using an ability. At 100% potential and accuracy, Nami and Karma can and will surpass her, but since you can't land all of your skillshots or always proc off the secondary effects of your spells and make it worthwhile because of the enemy team's reflexes or through your own error, Sona's consistency and simplicity stays superior.
Need it any more in-depth? No, I will not be providing a tl;dr.