I used Cosmos when I first started playing but it lagged my gameplay so much it was unbelievable. I wouldn't use it again unl;ess they have optomized it alot. I use Discord addons as they are the most customizeable, as you can see form my screenshot =)
Currently here is my list of addons I use, along with a screenshot.
AntiDaze (Takes off Aspect of the Cheetah if you get hit)
Atlas (In game maps of instances)
AutoRepair (Automatically repairs your gear when you can)
Confab (Better chat boxes)
CT_QuestLevel
CT_RaidAssist
CT_RaBossMods
DiscordArt
DiscordArtOptions
DiscordLibrary
DiscordUnitFrames
DiscordUnitFramesOptions
FlexBar
FreezeHelper
ICU
LootLink (Keeps tack of every item you ever come across in a database)
Nurfed_CombatLog (Easy to read combat log)
Scrolling Combat Text
SmartPet
Titan (The bar across the top)
TitanHonorPlus
We got our server first Benediction / Anathema for our priest. Its a new server so we had to farm for the Eye Of Shadow lol