i find that all supports can zone and harass quite well, i find that sona and nunu have the best luck at this though, i dont know if she has the best range, but your purple aura gives a nice slow to make harass a tad bit safer if you do have to get close..
in fact sona is my little elo train as i can easily zone both enemy adc and support out, making my adc very fat with farm, and i keep pinks in the river so we dont get ganked( dont just ward your little bush, drop a green in their tri bush, and another pink at dragon, you cant die now), do this and you will hardly ever lose
im not the best mmid laner, but check out my support games, all supports do well, but sona outshines..
edit: i did some number crunching (not much i passed calculus, enough for a lifetime there, i hate math) and it seems that lulu does more damage, if you look at her slow, and help pix adding damage, BUT, you shouldnt get all that agressive, while she has high damage for a support, you cant win a trade with corki, SO, if you arent trying to shove lane, sona and her Q power chord is (IMO) the best support harass in the game, as it doesnt go through minions, and if needed, it can be used to lower enemy adc damage (if in a close trade) or slow (if chasing)
once again, i gots mad love for sona