Addons that you cannot play without.
 

So I haven't seen a thread like this if i am wrong I am sorry but in the case that I am not.

Which addons can you not play unless you have them installed.

For me its:
Tidy Plates (helps me know who do and don't have aggro on when tanking)
Razer Naga Bar-mod (like domminos (I hate the blizzard button setup)
Postal (clicking each mail is boring lol)

OpenRDX

Pretty much does whatever I need it to do.

nUI, TitanPanel, AutoProfitX, WIM, Stopthespam, Badboy: AntiSpam, Bagnon, Mapster, Decursive, BigWigs, Postal.

I absolutely, without a doubt, can not play without Dash. It may not have been updated in 3 years, but it still does what I want.

Absolutely need PerfectRaid, even though it's sadly highly out of date and buggy at the moment.

Other than that, I need WhoTaunted and Crybaby, when I'm on my tanks.

OmniCC (About to try Tulla's other one, though)

A Square minimap, using PocketPlot.

Bankitems and Flashfind. (I'm an altoholic, and it's hard to remember what you have and where).

!Buggrabber and Bugsack, with a libdatabroker display, to tidy things up, if they pop up.

Cosplay, or something else for helping my dressup screen.

Not really an addon, but I couldn't last a day healing without mouseover macros.

Forte, Altoholic, ZomgBuffs, Postal, Clique, Addon Control Panel, ReforgeLite, Baggins, TellMeWhen.

Postal and Badboy

most definitely nUI. can't play for more than a few minutes without it.

OmniCC, Clique, NinjaPanel, Auctionator and Adibags.

The Best of the Best:
GTFO is such an awesome mod.
DBM of course.
TidyPlates (ThreatPlates added is nice too)
Dominos

Others that I like a lot:
Carbonite
Chatter
NeedToKnow
Kong

I use a bunch more, but those are the best of the best and make a big impact on the game for me, off the top of my head.

Unit frames: Stuf
Bar mod: Dominos (but Bartender would work as well)
Chat: Badboy with CCleaner and any of the regular chat mods (currently using wChat)
Other: Bindpad for directly binding my WoW mouse, profession secondary spells (like milling), etc.
LitePanels or kgPanels (most importantly to automate my UI)
and a 'tweaker' addon (used wIn1 until recently. Trying out Leatrix Plus now)

I can, honestly, not play without oGlow. I just can't do it.

Atlasloot, must find all my shiny transmog gear.

I've been using this: http://www.wowhead.com/transmog-sets :D

best addon.... chatbar was typing /g to talk to guild? not anymore

ooohh...

Quest Guru
Atlas
Altoholic
GupPet (pet/mount nut but cant sand rideing same one => )
Mik Scrolling Battle Text
:banana:

DBM
Badboy
Class Icons (outdated but still works for me, blizzard should implement this)
Who Taunted? (awesome addon)
Instigator (Tells u who broke the CC and how)
Tidy Plates: Threat Plates
OmniCC (cooldown counter)
Carbonite
SmartBuff

SpartanUI
DBM
Carbonite

I have been using Spartan since its inception, when the patch first hit and I could not get Spartanui to work right I was lost. Sik UI was the closest I could come to the feel of Spartanui, it is very good but it was not what I was used to. I am just glad a couple of users found a fixes for Spartan.

essential
evl_SliceDice
LoseControl
rFilter3

nice to have
_Corpse
EnhancedAchievements
TargetOfTargetToolTip
ToolTipAnchor
tullaCooldownCount
tullaRange

(:

Necessary
ArkInventory [I haven't found an inventory mod that works as well for my needs as AI over the years I've been playing]
Some sort of bar mod [I've used Dominos and Bartender over the years, as well as a few others. Just about any bar mod is better than the stock bars]
oUF [As much as it may be a pain to make/modify layouts sometimes, I've found no other unit frame mod to be as customizable as oUF. I can always make, or modify a current layout to fit my needs without too much trouble]

Preferable
VanasKOS [I've been rolling on Arthas lately, and it's nice to keep a list of people who camped me, so I can camp them back. It also sends my list to my guildies if they run the addon, so they can steamroll anybody on the list if they come across them. :)]

Lately I have been using:

ShestakUI
NPC Scan
Pawn
Miniloot
TinyDPS
AdiBags
Tell Me When