I have to disagree. Her bubble is an incredibly unreliable stun and that can be avoided with summoners or mobility spells. Taric has a reliable stun that can't be dodged...
625 Range vs 875 Range. It has flight time just like Aqua Prison does so you can still position to neutralize the stun (though, yes, it will still land).
And if someone uses Flash to dodge her 14s cooldown stun, I'll take that as a win.
...and massive armor shred/and bonus armor with his W whereas Nami is very squishy even with proper items.
And Nami adds bonus damage (+25 Magic Damage at Rank 1; that's 500 Gold worth of AD) with a slow. Hence why I said more offensive. She harasses and pressures better. It's also at range. Taric's is an all-in at melee range which limits his effective range a lot.
Nami's heal is also pretty bad in that, it can only bounce if you're near an enemy champion. Otherwise it will only heal one target. Taric can heal both himself and another target and increase the effectiveness of said heal if he only uses it on himself.
Position for it then. You should almost never have Ebb and Flow JUST heal. It should at least heal and damage. Taric's heal does scale better (+40 per Rank vs +30 per Rank) with far better scaling (+0.6 AP vs +0.3 AP) but it also has over twice the base cooldown (20s vs 9s) and Imbue isn't maxed first compared to Ebb and Flow.
Nami requires more to play than Taric (skillshot stun, positioning heal/damage) but she offers a lot more pressure and zoning potential than Taric does. She's really good with Caitlyn (stun + trap, her damage buff) and ADCs with a good amount of burst (Draven, Varus).
Edit: I don't think she's one of the best supports (Lulu, Sona, Nunu take that crown for the most part) but she's still one of the better options alongside Taric, Zyra, etc.