what is the most effective heals for pvp? mostly rated bgs

Well, I think all healers are perfectly viable to pvp. pvping is all about knowing other classes and being an expert at your own. if you're about to start pvping as a healer, i would start with whichever current healer you feel most comfortable with.

As to your question. Personally I play a paladin, I find that I excel in BGs but I'm very poor in Arenas. However, i think is mostly due to the fact that my pvp gear isn't too great. In arenas, you need to have enough resciliance so that when you get double-teamed (which will ALWAYS happen) you don't die within 3 strikes. The best healer I've seen in pvp is my cousin who is a resto druid. He knows his class so well that is REALLY hard for any class to kill him in an arena. Yet in BGs I usually do much better than him.

So best heals? all classes provide good heals, I'd say HoT classes are better for 2v2/3v3 while casting classes are better for 5v5. In BGs is more a matter of the dps not letting you die which unless you're running with friends, well...good luck with that lol.

Here is the key thing I gleaned from your question: Rated BGs. This new pvp style is a whole different beast and comparing it to regular BGs is a poor comparison once you go up against well prepared teams.

Couple of questions for you:
1)What healers are currently or most likely in your rBG group?
2)Do you have previous healing class experience do you have?
3)What will your healing role in the rBG be?
4)Play style

As for 1) having 3 resto shammies is just a horrible idea in a rBG set up. Diversity diversity diversity. All healing classes are viable in rBGs but there are some niche roles and depending on the battle having the right class for the right scenario will have a big impact on your performance. Don't do what my guild did and bring 3 priest *facepalm*

In regards to 2) Start out with what you know. I simply cannot heal well on my resto shammy in a bg/arena setting. My main is a preist and he has a full set of BT gear and I feel comfortable in all 3 specs in a pvp scenario. Guess which one I'm bringing to rbg night?

3) is a complicated question. A resto shammy is awesome in being a healing turret in BGs like arathi basin and to some extent wsg where there is a lot of cover. However if your role is to go with the mobile group on the attack or running with the flag carrier a far more mobile class (ex. Disc or Druid) has a lot of benefits to it over shammy though I'm sure some shammies will disagree with me. Paladins have a mix of both mobility and strong healing turret ability, on top of that they can take quite a few hits too.

4)How do you like to play. Do you like being a class that has huge heals and can keep a group of pvpers topped off for a REALLY long time? Would you rather be able to control opponents actions and reactions better? (Personally I think shamans are the king at this do to slowing totems/hex/wind shear/Ice Shock/grounding totem/etc) Or would you rather kite and keep healing while on the go like a druid that toggles travel form and caster form? Or would you like to be able to put the hurt down if need be like a disc priest!

Stupid wall of text. Just remember rBGs are a whole different beast then pug BGs. THE BEST of the best will be in rBGs so don't get discouraged, watch some rbg videos in hi def and see what other healers are doing.