Ok Think the best way to decribe/list what I feel is essential is with an illustrated pic first.
Though most of my mods are all ACE plugins (listed as ACE), some are not (listed as NOT ACE).
Also, some of the mods that are essential are not visable in the pic, so I will explain them after the pic's breakdown.
So... getting to it... here is what I use in Raids/AV:
1. Omen (ACE)
Aggro meter that everyone MUST have for raids. Not only does it let you and everone know how bad your aggro is, but also warns you when you are getting close to pulling aggro off of the tank.
2. ElkBuffBars (ACE)
A handy highly customizable mod, that makes it really easy see timers on all your buffs/debufs. You can set everything to show up in 1 bar or many different ones. Because this mod takes a bit to initially set up and customize (mostly learning what menu does what) , I don't recommend this one till you have time to fidgit with it.
3. TipBuddy (NOT ACE)
I love this mod (mad addicted to it). Its extremely handy in pvp. As you can see in the pic, my cursor is hovering (and I did say hover, you don't have to select anyone for it to pop up) over Magelena and a box pops up showing her name, class, health etc.
Often (esp. in arena) in pvp, you need to find out what horde is what class quickly and selecting each one takes waaaaay to long.
This mod works well in conjunction with MobInfo-2 (NOT ACE; explained below)
. The developer who created TipBuddy, no longer supports it and has created TipTac instead. Tipbuddy still works so I havn't bothered to test TipTac yet.
4. Trinket Menu (NOT ACE)
This mod creates 2 movable icons, representing the 2 trinkets you are wearing. If you open the trinket menu you can setup a quick key to auto use a trinket (for instance I use the / key to use my pvp trinket and break out of things in combat). The mod also lets you change the trinkets out on the fly for any other one that is in your backpack, via a designated key (for Instance I use SHIFT + left/right mouse button; the left mouse button changes the first trinket out and the right changes out the 2nd trinket).
5. SimpleMiniMap (ACE)
Handy Mod for moving and editing your minimap.
6. AutoBar (ACE)
This mod creates a customizable new bar (outside of your normal blizzard bars) that is specifically designed to auto list every single consumable that you have on you at any given point (ie. food, water, potions, healthstones, mount, poisons, etc; even your hearthstone if you want). It ONLY lists what you have on you and you can filter out what you dont want to show up. By default it lists EVERYTHING, so I recommend filtering some things out or you will have a giganticsour of a bar across your screen.
Personally to keep the bar small and useful, I only have it list what I commonly use in combat (ie. Health/Mana potions, bandages, healthstones and poisons).
7. Bartender3 (ACE)
Replaces the normal WoW bars with customizable/movable bars. You can edit how many buttons are on each bar, how far apart the buttons are from each other, the size and width of the bars/buttons, even change the direction and shape of the bars.
CyCircled (ACE; explained below)
is a nice mod to use in conjunction with Bartender3.
8. AG_Unitframes (ACE)
EXTREMELY customizable/movable unit frames. Wait, you don't know what unit frames are? You do and just don't realize it. The frame in the upper left corner that lists your name, health and mana/rage/energy? That is a unit frame. When you select something else and a new box opens listing what you are targeting that is a unit frame. There is a bagillion different ways you can set up your unit frames, so I will instead just give you a quick on how I set up mine. When in a raid my party frames are hidden (I usually have them where grid is at the moment). Also the frames are colored by what your classes is (ie mine is white because I am priest and Magdelana is brown because she is a warrior). I have it so opponets are listed as red and their names are colored by what their class is. Soo, as you can see in the pic, I am targeting Magdelena and she is targeting a horde mage (the bar is red and the text is blue).
is another GREAT unit frame mod to use (instead of using AG_UnitFrames), but is a lil bit more of a resource hog. If your system can handle it, I recommend testing them both out (separately) to see which you like better.
9. Grid (ACE)
This mod is a highly customizable raid-frame healers dream (esp. in AV PvP)! Not only does this mod list every member in your raid, but has a plethora of things you can track on each person. For instance, the mod can track their health, whether or not they are in casting distance of you, if they have aggro, if they have an affliction, who all will be affected by an aoe heal, etc (the list goes on). Classes are sorted by color and are all in a tight little handy dandy unit frame box.
10. Decursive (ACE)
All classes that have a means of removing debuffs from other people should have this mod. Much like Grid, this mod lists all members in a party/raid. They are sorted by group and class (just like in grid as well). The biggest difference is that Decursive lists everyone by a small dot instead of a unit frame. When ever anyone is afflicted by something you can remove and is in range, the dot lights up. Simply clicking the left or right mouse button auto casts the appropriate removal spell. If your class can remove more then 1 type of affliction the colors will light up differently. For instance, my priest can remove magic and disease. So if someone is in range of me and has a magic debuff on them it lights up red(and I left click on the dot), if they have a disease it lights up green(and I right click). You can also use the mod to set up macro removal quick keys if you like.
11. MetaHUD (NOT ACE)
I find this mod extremely useful for melee type classes. It creates a HuD reticle around your character. Listing both you and your opponents health/energy/rage/magic etc. It also tracks things like combo dots and range. If you use quick keys to attack with like I do, using this HUD makes it so you never have to look away from your opponent, as all pertinent information is wrapped around your character.
12. Fubar (ACE)
Essential Mods Not Shown But Running In the Background
(*I will add the rest shortly, its a edit/work in progress*)