I've won plenty of games as taric. Used to and still do some times wreck lanes with all ins at level 3 and 6. Your still not swinging me. Zyra offers nothing to her adc, in fact she's even more reliant on her adc, than leona or taric. Sona is the most easily bursted supp in the league. I think she's strong, but make one mistake in your positioning and your dead.
I think the best supp's in the league right now, regardless of rank, go
Nami
Morgana
Blitz
Thresh
Not in any order. I just think the most successful bot lanes use these champs. My original point was that supp is one of the hardest positions to play, as you need to rely on your adc being halfway competent. This is completely different from the other lanes. You need to force feed your adc and only the best supp's can go from bronze to plat doing this alone. Hell up to a point, I would say if you are confident in your skill you should just take the kills for yourself and carry your way out.