The addons I use, in no particular order:
Auctioneer - an awesome tool that scans and records pricing trends on the auction house and also makes the auction UI a bit more user-friendly.
Bad Pet - whispers hunters and warlocks when their pet has taunt on; great for tanks.
Block Chinese - the Chinese font in this game is a lot bigger than normal text, and I'm used to servers with high Chinese population spamming trade chat. This blocks all Chinese text/posts; not like you can read it anyway.
Clique - click-casting interface that can go with most raid frames. I actually prefer Vuhdo for healing but haven't set it up on my new computer yet. Custom raid frames + click casting + healing = easy mode.
Master Plan - Garrison addon for missions and ship missions; auto-picks the best follower combos.
Handy Notes - map markers for Draenor treasures, incredibly useful.
OPie - INCREDIBLY useful raid marker addon. Brings up a circular menu when a hotkey is pressed for ground and target markers.
Postal - like, THE mail mod. Auto open, delete all checked, etc.
ElvUI - my UI package of choice. There are a lot of UI packages TukUI, RealUI, LUI, etc, and in my opinion they're a lot better than totally self-made UIs. They usually take up a lot less memory and don't bog down the client as much. I can help anyone customize their UI to their liking either way.
DBM - Deadly Boss Mods. If there is one mod we require it should be DBM, hands down. WoW is pretty good at announcing mechanics during fights, but DBM takes it to the next level by adding in timers and countdowns for raid-wiping abilities or phase switches. This is a MUST-HAVE.
Weakauras2 - Buff monitor system; it's a bit complicated to get going but once you get it working it's amazing.
Tell Me When - Similar to Weakauras, lets you know when things happen. Can use it for cooldowns or procs.
Need to Know - Similar and a little bit easier to use than Weakauras, but less customizable. Basically puts bars in where you can monitor cooldowns, buffs, enemy debuffs, etc.
Skada - Damage meter, healing meter, threat meter, everything meter. I personally think it's better than recount, however it takes a little bit more to set up correctly. Either way, having a DPS or healing meter is good to gauge your skill and improvement with more gear/more refined rotation.
This is a small list, but what I've been using for awhile.