I play a D/Wd/S, rerolled Warden from Mage. As far as I could tell, D/M didnt really add much to my ability to heal well and not die. As said above, D/S is probably the best healer in the game, but my personal preference is D/Wd.
D/Wd comes with briar shield (around +66% def if you want to up it to 77 o-o) and some other FANTASTIC buffs. The Enhanced Briar shield is nice to put on others, and I've had people tell me it's reflected as much as 50k (maybe more). Others won't get the defence, but it's still useful for you (less damage for you means less time dead. As a healer, you can't heal when you're dead). At the 60/60 elite, it makes your heals stronger to anyone you put it on, and whenever the shield's reflection is triggered, nearby people will get healed. Also, the 70 elite gives you something like a priest's soul source. Mysterious Grace is a nice buff for your heals, and any mage friends you have will love you for the extra mdam

Extended Recovery messes with Recover so it gives NP and makes the HoT tick 6 seconds longer, but also heals a little less each tick. The 50 elite isn't really anything to write home about, because it's a %heal, most people won't get a lot of use out of it. Grace of the Forest gives you 10% extra mana and 6% extra healing if you have any NP.
Now for the not so good ones: Essence Extraction gives you 4 instant NP for 10% max mana, but I don't find I use it or need it too much in a fight. If you're using Recover, the elite will make sure you have NP most of the time. I guess this one is ok for saving like 3 seconds of charging up NP before a fight. Avalanche makes your Rock Slide do more damage, which is nice, but if you're healing, doesn't do much for you. Healing Ripple is a skill that heals your target, then bounces to a nearby person healing for half the amount your target was healed for, then heals a 3rd person (or bounces back to the original target I think if only 2 people) for half the amount the 2nd person was healed for. It's ok, and I'm probably just using this one wrong, but I really don't like it too much. It doesn't always get who you want it to and the heal is less for each person after. Other than that, it's pretty much like Restore Life.
Long story short: Warden is your best friend. Go Elves!
And from my server's resident Wd/D, Wd/D isn't too bad of a tank either and is great for holding aggro. It also gives some nice personal buffs

Wd/S and S/Wd are nice too from what I've seen (Chiron anyone?).
Of those 6 possible combos (D/Wd D/S Wd/S Wd/D S/Wd S/D), the only one I wouldn't recommend is S/D. Because it sucks o3o Hard.