Depends on your playstyle and the kind of content you'll be doing with the class. Let's take a look at the options.

Crit - Will effect both heals and dps, giving an increase to both. Can cause a bit of overheal, but not the end of the world if it does. Besides, seeing a Cure 2 turn into a Bene is
Det - Flat baseline increase to both heals and dps, though you need a lot to see a little difference.
Spell Speed - Will give increase to hots, and if you play a reactive healer, it makes your casts faster. However, it will cause you to run out of MP faster and since you should be precasting anyways for outgoing damage, it's not as valuable as many think.
Direct Hit - Increase to DPS only. If you're going for those orange parses, this will help.
Piety - Can be helpful for progging fights since you're casting / ressing more but WHM also has strong MP regen and you need a LOT of piety to make a difference.
Vitality - Right Side only. Can be helpful for prog and gives a bit of wiggle room when learning on mechanics.

Personally, I have over 2700 on my WHM with raid food on and I'd stack more if I could.
Crit > Det > DH for me and I avoid SS like the plague on gear and melds. Pie to a comfort level, though you can get enough on gear that you shouldn't have to meld. Vit melds on right side for early Savage prog.

But ultimately, any melds are viable if they work for you.