Assuming at least 35 mending or so.

The way healing works here, you're looking at a tradeoff between power efficiency vs HP per second. An easy rule is "the smaller the heal, the more efficient it is, but with lower HP per second". I'll make a little chart to illustrate this, based entirely off delve info with no modifiers at 35 Mending.

Heal ------- HP per power - HP per second
Minor ------ 8.7 ------------- 64
Major ------ 8.0 ------------ 120
Greater --- 6.3 ------------- 174

Now, your group heals have the potential to give more HP per point of power, and per second, if they are healing enough targets. Spreadheal is really excellent at this patch level, and is efficient enough to be ideal in a wide range of situations. However, the base and spec group heals are only useful if 5 or more members of your group are equally damaged.

Always better to use group heal if more than 2 targets need hp.
Then it is a question of what is the priority:
- do you want everybody to be at max hp?
- do you want to quickly heal only 1 person?
- do you want to heal in a few seconds somebody for a lot?
Basic healing questions you ask yourself.

There is not a simple one fits all heal that you would just spam in any situation.
Just keep in mind that heals will receive a variability.
So a lvl 35 heal with no variability (35 mending) might be better for its consistency than a lvl 50 heal with high variability