A lot of different people have a lot of different opinions about what stat they find the best. It seems it is somewhat dependent on your job (tank healer / raid healer / heroic 5 mans / etc), on your personal preference and also largely the situation.
That's (IMO) quite a good thing as it means not everyone is a clone of the next person.
I personally prefer to just tell people what each stat means, and in what roll it's good. If people just dump A > B > C > D it doesn't really say that much, people often don't learn to think either; because is there some kind of dropoff point where B > A ? etc
Generally speaking; Intellect is your best stat. It gives a ton of benefits, from increased spellpower, to increased crit. From increased mana pools, to increased mana regen (since you have a bigger mana pool you regen more through buffs like Replenishment)
So in most cases you want to gem for intellect, or int/spirit or int/crit or int/haste; etc.
Spirit is a stat that is, in my opinion, somewhat comparable to what hit is for dps'ers. It's one of the most vital stats up to the point where it stops giving benefit. For a healer that "cap" is a bit of an undetermined one and a really dynamic one. Because spirit gives mana regen. And mana regen is the best thing you can have if you run out of mana, but useless if you don't. That goes for spirit as well. A healer out of mana is a useless healer, so spirit is probably the strongest stat when that happens. But if you end the fight with 40% mana, or without having used cooldowns that spirit could have better been reforged to a different stat.
Since Intellect is independent from ratings on most things. By which i mean to say, that often you don't chose between either intellect on a rating; but between ratings amongst themselves. If you get a piece of gear there will be intellect on it regardless. You also can't reforge a rating to intellect for example. Gemming is an exception to this, which is why you often gem for intellect or a combination of intellect+rating or intellect+Spirit.
But since intellect is independent the most common question is which rating is strongest. And that largely depends on preference and what job you do.
Ive seen shamans swear by stacking haste, others prefer to aim for crit. Mastery is generally considered a lesser stat, but it still has its strong points.
Just to put things in perspective:
Haste is often seen as a high priority until at least 916 rating. The reason is that:
a) That gives you an extra tick on both your riptide (at 261 rating already actually) and on your earthliving
b) And it's relatively easy to get at this point. Unlike say trying to get a 2nd extra tick of riptide which is at 2000 haste rating. And a second extra tick on earthliving is at 4000 haste rating.
Haste also is a favorite of many of the shaman who are going to raid heal. The reason is that haste is by far the best throughput increasing stat. Especially if you're using a lot of chain heals.
Crit synergizes really well with a lot of our abilities. Not only does it give you a chance on extra healing. It also procs certain stuff in our tree. For example Improved Water Shield, which makes crit also good for mana regen and sustainability.
Since there is less chance of overhealing in cataclysm (mostly because a tank can drop a lot lower before it needs healing and doesn't require spam healing) the critting of a heal has regained some of its lost glory.
Quite some people prefer crit over haste, especially if they are tank healing. Having a high uptime on Ancestral Healing and the fact your heals are very efficient when you crit (not only doing 150% of healing, but also reducing the cost because of Imp. WS), is something a tank healer is more interrested in.
Not very experienced with this. But this stat has been much much debated. On one hand the effects are rather small if you heal a target at 70%+ HP making it a lesser desirable stat.
On the other hand it is there when you need it most and it synergizes well with crit for example. Getting a crit heal on a low person when you got some mastery gives a lot of health.
If you can permit it you can even adjust your playstyle slightly to get the max out of it. If you know the encounter you can let the tank drop to roughly 30% health before you heal him up with a big heal.
What stat is best.
Again, depends on your playstyle and on what you need at that point.
Are you a raid healer running out of mana?
Spirit might be your best stat followed by intellect and haste.
Are you a healer currently mostly working on heroics (which i am too), without too much mana issues, then you could go with:
Haste (till 916) > Int > Crit > Mastery > Haste