Sona is flexible, like Morgana. Morgana can zone and poke, she can stun/slow and she can shield her adc from spell effects. She can wreck lane with the right adc and she work well with just about any adc.
Sona can heal, speed up to peel/engage, poke damage and has a beefy stun as well. Her passive spellchords are more art than a science but damn when you get a sona that knows how to work those babies in right. Her cons are that she is super squishy, fairly mana heavy if you're just a spammer, her heal isn't really super strong like soraka, her only REAL cc is her ultimate and the adc has to know the auras to really respond to them correctly in the situation. Sona takes a bit of communication to really shine. She's a jack of all trades.
Soraka is super strong if you max W first. I always max it first. The slight hit to my HP is compounded towards those intense heals. It's a real force multiplier maxing w over q first. Her global heal and her e silence is also very sought after. You can basically totally shut down a Leona from diving with well timed e's.
Nami has been a personal favorite of mine recently. Great poke AND a heal that bounces all in one spell, a dmg buff for three hits, a speed boost on every skill cast on an ally, a hard cc bubble q and an ult with a long range and knock up WITH a speed boost for added bullshit.
Want something purely defensive and you get braum.
Want hard engage you go with Leona and blitz.
You want to watch a clusterfuck of monumental proportions then you watch a good thresh with a bad adc that doesn't know what a lantern is aka Thresh Tears.
Every support has a certain niche they fill or are an all around champ that works with most comps and a preference for particulars but not limited to them.
So saying one is better than any other or is "best" is basically non-existent. They're all my favorites.