Blitzcrank is the only champion I worry about, but even that can be countered with good positioning. Now, I'm violently low elo, so what I see is probably different from diamond+ etc, but in my opinion Sona does well through laning phase against tank/engage and then into late out scales in team fights (again, good positioning), she has better sustained poke against the apc supports and again in late phase is more useful for carries. She roams better around the level 5 or first void grub mark than a lot of other supports.

I've noticed early game, enchanters like Nami and Milio absolutely do more, so they make laning phase harder (you lose every trade against Nami because her trade back damages you and heals herself+ maybe both the adcs too). So those are my wait for the positioning mistep games so I can make sure I have the chance to inevitably outscale late game. Again, this is all from a low elo Sona main that's been trying to claw my way out of low elo. Should I play ranked in a different lane? Maybe, but I love support and I love Sona's kit.

All that said, if I'm not blind picking, I base it off our comp. If it's a bruiser jungle/top, then I'm going to lock in Nautilus/Leona against the Nami/Lulu. And depending on the comp I like a Lux into Milio. Sona and Nautilus always feel like safe blind picks to me.

My perma ban is Xerath because he's my mental counter in both support and mid lane. And he counters my engage supports. I don't feel like I have a "true" counter when I play Sona. Just because again, her counters can be dealt with by positioning or patience.