For shadow, Affdotspriest and Gnosis are essential. A good nameplate setup is also crucial, and while many use tidyplates, I find Elv's nameplates are great as well. Also, a configurable boss unit frame UI so that you can track your dots on multiple boss frames. And another chime for weakauras, which is essential for anybody playing WoW these days.
For healing, Grid+clique (browse grid modules to get it nicely setup) and ofc weakauras get the job done very well.
I don't know exactly what TellMeWhen does, but the two biggest perks to WA is that it allows you to track how many PoM stacks are left and Rapture's CD. It's also really customizable, so you can tailor its appearance to your liking. You can also add sound warnings to certain abilities that are important to track.
Finally, you can also use WA to track specific boss debuffs, abilities or casts. I've found that really helpful when it comes to raiding.
Vuhdo and Weakauras, like many mentioned above. They both allow you to import settings from other players, which will save you from having to set up everything from the ground up. A Google search will help you find priest Weakauras others have been kind enough to share.
The OP is a healing priest. The only add-on I would say is "required" is BigWigs or DBM. Healing is all about watching boss timers/knowing what is happening and when. Blizzard's default raid frames are "OK" for healing.
If you're looking to switch to new raid frames though, or even just looking to try something out for a bit, I went from Grid -> ElvUI raid frames in 5.3 and haven't looked back.
- No more manual entry of boss debuffs (yes I know GridStatusRaidDebuffs exists but it's like never updated and is unreliable a lot of the time)
- All raid cooldown "buffs" automatically shown
- All tank cooldowns shown
- Many more sections of the frame to customize (akin to GridIndicatorCornerIcons but with less fuss)
- Absorb and healing estimation
Can't beat the all-in-one package that is always up to date for every patch (great devs).
I just recently discovered Shadowed Unit Frames. I am IN LOVE with it. Simple, light, pretty, VERY functional.
With that, my current setup goes with:
Shadowed Unit Frames <3
Affdots + Affdotspriest
Decursive + Healbot. I know, it sucks compared to grid + clique; but old habits die hard and the sole idea of configuring deters me from changing.
A bit off topic, but has anyone experienced massive Lag issues with grid lately? I have the latest update (which is not updated for 5.4) and everything was fine up until a week ago. Have dozen plugins which also weren't updated in a milllion years.
I'd simply say don't crowd your screen with add ons. It would be easiest if you knew what information you need and got the add ons for that.
In terms of most useful to heal?
Weakauras is probably ideal for customization. You really shouldn't need to use much more than wa along with your raid frame preference from a healing perspective. Inb4 DBM and all other 'non-healing specific' add ons. In terms of just healing, keep it simple.