I realize this thread is primarily focused on addons, but there's also something to be said for not using any addons which should also at least be mentioned in an addon thread since there are those who actually think it isn't possible to heal efficiently without an addon.
To each their own in regards to addons in my opinion, but it is
possible to heal, even at the high-end without
I've healed 11/12 Heroic ICC25 and Heroic RS25 up to 45% before we stopped raiding, all fights with and without using an addon such as Grid, VuhDo, Healbot, Grid+Clique, etc.
The default UI frames have plenty of information for many people. So long as you know your class quite thoroughly, the encounters, and pay attention well and have a good memory. It tells me if they have a debuff I can get rid of, tells me health missing, I can scale quite easily to whatever size I want, shows very well other incoming heals, and is the simplest to setup possible in terms of the UI itself.
Couple those with efficient and decently robust macros such as:
/cast [@mouseover,help,exists] [@target,help,exists] [@focus,help,exists] [@player] Riptide
One could make any numerous amounts of additions and changes to the macro to increase functionality and robustness, that's just the template I use for all of my heals on all of my healers and it works perfectly for me.
Make sure to have efficient key bindings that fit your hand and what you're used to and capable of using efficiently and quickly, and with those, you'll be capable of keeping up with any other healer using any healing addon.
It's more a matter of what you're used to using and can make the best use of than which addon is best. All of the "big three" healing addons can be configured near identically to the point where there is only minor aesthetic differences and very minor usability differences that really don't make a difference overall.
Addons aren't required
, yes they can be very helpful and very useful, especially for those who need the addon to help them track stuff instead of reading DBM and being able to remember things, not everyone has that capability. This is all fine; addons are great for helping with that, but the alternative to addons should be represented as well.