It depends on what you want to add to your team. I would personally go with fire or sonic. Sonic because it debuffs resistance a good bit and Fire because it has nice Single target and AoE damage.
If you plan on playing a purely support toon though, then you should Probably go with sonic imo.
I have a 34 elec/pain and a 39 fire/pain.
So far the fire corr has been more fun.
I have a 36 elec/pain and a 36 fire/kin and I can say that for me elec/pain is a great toon to play...I enjoy him a lot more then my fire/kin whom I have actually retired.
I too play with two other people on a regular basis and even with elecs lower damage over all....the end drain is nice to add to a team.
Thanks all for the feed back. Fire is a lot of fun. It's the primary on 2 other cors I have. Since I've been wanting to make an elec/ cor maybe I will go with elec/pain. 
If you do make sure to get stamina it has helped in my experience.
I also ended up making two simultaneous builds, one dmg focused and one team focused and both have been fun.
Well since you do not PvP, and already have tried Fire/ it might be worth giving Ice/ a try. It has nice fast animations that you can fit in between using your secondary without missing a heal, etc. It does great single-target damage and has a great fast single target hold for a little control. The slow effect sometimes helps too. For AoE damage it has 2 big hitters as well.
If you want to concentrate on your secondary and support, the Ice primary works quite well. It probably has the best single target damage also. Personally, using some of the slow animations of Sonic/ may make it harder to use your secondary because you will not have quick reaction time to your tank getting damage spikes or a squishy that grabs aggro unexpectedly.
I have a level 41 Dark/Pain that I like a lot. Dark/ is nice because it has Tenebrous Tentacles which locks down mobs. This is great solo or teamed. And the debufs Dark/ provides are nice too. Two big cones gives it some decent AoE performance.
Dark/ is also really loose. I took the first two attacks and the two cones, and I'm pretty sure that was it. /Pain you pretty much have to take every power, so Dark/ is a nice complement in that respect.
For those with /pain, did you take all three powers: Soothe, Share Pain and Soothing Aura? And why or why not? Thx!
For my pain i took everything but share pain. With the WoP buff easily perma i can heal what i need with just the AoE heal and the target heal without needing to damage myself for a masive heal. Soothing aura is nice just to keep you own hp full without having to use the AoE heal. Thats my opinion on those powers
Virtue
--Blazing Tiger-- 50 Invulrn/Fire Tank
<<Virtues Tankiest Kitty>>
Try my Arcs: #4892 and #112548
@Blazing Tiger and @Aqua Fox
I took absolutely everything in /Pain.
1. My primary is Dark/, which is pretty loose, so I had available power picks.
2. More healing is more healing:
3. Soothing Aura heals a little over time. It's a great downtime reducer and will automatically heal lesser damage so I just concentrate on the big stuff. It nearly doubles my AoE healing (do the numbers) and doesn't require that I mash a button.
4. Share Pain is my big heal. When someone takes the alpha and you have to bring them back up fast, Share Pain will do it faster and better. Plus it's a self buff, so why not? Moar damage, pls. I took this early and slotted the heck out of it.
5. Soothe is the stop gap between Nullify Pain and Share Pain. I can use both Share Pain and Soothe on the same target to quickly heal a lot, or I can just use Soothe (which has no side effects) to top off teammates. I slotted Soothe just behind Nullify Pain and Share Pain (and I took Soothe later too, in my 20's. Share Pain I took right away.).
I want to roll a /pain corr. What would be the best primary and why? I never pvp and usually team with 2 - 3 other players.