As a little preamble, I'm speaking from a very casual perspective, so please mind what I say.
I've tried stacking all three stats, and I've found some interesting results that might be worthwhile to share:
stacking haste is the best for when reaction time actually matter more than anything, and is the best for when an increase of throughput is needed all the time, ie you have a constant aura that won't let players get too low on HPs, as if there are many healers putting out substantial amounts or AoE heals, and if you need to look good on the meters, it's the better stat for sure, as you reduce the risk of others sniping you.
Mastery on the other hand, is very nice for doing HPS that actually matters, as you will heal a player more the more he/she's injured, and it's also the mana neutral stat in the equation here, as it's an increase of throughput that will not affect your mana pool but for the fact that you will need to cast less to top a player off. I'm not experienced enough to say if it's best for heavy AoE settings, but my gut feel is that it's best for spiky damage, as the TC+GHW mechanic really favors us there.
Crit as you probably know it's both an increase in throughput as it is for regeneration, but since it's really RNG dependant by its very nature, it can lead to mixed results, especially if the extra healing (it's 200% + 60% on another target from AA) goes wasted, which is a real possiblity. But if it doesn't, it's a pretty linear throughput increase. I'd like to point out one thing though, stacking crit is unreliable, but if you get enough, it starts to smooth out. I'm not geared enough to test this, but I'd really like to know if at high levels of gear, it's possible to get to a level where stacking crit is actually worthwhile.
When comparing the three, and especially mastery and crit, you also need to keep in mind that crit requires lots of rating per 1% increase, which is why mastery stacking is usually favored in raiding environment (for what i know at least).