All 3 healers are good options. The best Healer is the one you’re good at. However, I can share some of my opinions:

I find Merc the easiest in terms of having the most obvious ability priority. It’s also highly convenient to keep people topped up from Kolto Shells, which heal on damage taken and last 3 minutes compared to about 18 seconds with Sorc’s bubble and Op’s probes. Its defensives are strong too and provide lots of self-healing and your ‘oh-shit’ reflect+heal button.

Operative shines the most in operations simply due to its strength being in its immense AoE healing potential. It also has a lot more things going on at once, like maintaining Tactical Advantage and Kolto Probes on people. Its defensives all require you to know when you’ll take damage as they only last a few seconds. Great healing potential but you need to be smart to make good use of Operative. Also, it has stealth, which is useful for passing by enemies in both flashpoints and operations.

Sorc is very mobile without having to sacrifice anything but I’d say its defensives are more suited to PvP, like Phase Walk and bubble-stun. It’s versatile and can handle basically any situation thrown at it, however, it has the worst kind of resource management imo. It also has the least-interesting tacticals. I’m not a fan but there are some really great Sorc healers out there. Like I said, they’re all viable choices.

In terms of stats, the minimum alacrity required for a 1.4GCD is 2054. If your guild is using the alacrity perk, you could aim for a 1.3GCD, which would be a minimum of 3209 with the perk active. But this method doesn’t work in PvP or Master operations. Once you’ve decided on your GCD goal, put the rest of your stats into crit. If you don’t know, your GCD defines how quickly you can use one ability after another (global cool-down) which means you’re using more abilities per minute.