i wouldnt reccomend using anything else besides aesthetic addons. i used to use a lot of addons in the past, and i've been weening myself off of them steadily. i'm not saying using no addons makes you a better player, but if you can develop the awareness of things happening in the game without an addon telling you about it (literally in the case of gladiator lossa) then i think you will be a better player because of it.
i think its good to be aware of when your enemies are using certain cooldowns before they use them , rather than becoming aware of it when they do it. and its always a good thing to be looking at the game, instead of tunneling supposedly relevant information on your ui.
i like tidy plates and plate buffs, but i wouldnt call them mandatory or absolutely necessary for me to play effectively.
BGs: BattlegroundTargets, OmniCC (CD Counter), MikScrollingBattleText, ClassPortraitsFinal (Shows the Class Icon instead of portraits in Player and Target frames)
Arena: Gladius, GladiatorloSA, OmniCC, MikScrollingBattleText, ClassPortraitsFinal
DK Specific: UmberRunes (Lets you move your Runes and scale them wherever you want)