I know every player has their addon preferences, if any, and there's no real right or wrong setup. As long as it works for you, use it.
I, myself, have a pretty average setup. I'm still tweaking what I use from time to time, but for the most part it's pretty solid for what I need.
For raid frames, I use a combination of Grid and Xperl.
Grid takes your entire raid and puts it into what my guild leader refers to as "sanity in little boxes." The boxes are color-coded for each class, and display partial names of the players they represent. When a player loses health, the square boxes turn into rectangles. This becomes a game for myself and my guild leader (a holy paladin), where the goal is to turn those rectangles back into squares. This addon is also compatable with clique, which I will discuss later.
Grid also allows you to add in buffs and debuffs that will appear as different colors on the boxes. For example, I added in the debuff Weakened Soul as a light blue outline around the boxes. If a tank doesn't have weakened soul, I'm doing it wrong. It outlines who has aggro in red, which is helpful if you're in a two-tank fight, or if someone who shouldn't have aggro gets aggro. You can add in boss debuffs as well (such as Ignis the Furnace Master's Slag Pot), which helps to figure out who needs healing and when.
Xperl is just a rehash of the standard interface frames, but are, in my opinion, much cleaner. You can also choose where to display your buffs and debuffs (next to the minimap or under your heath bar). It's just a neat little override for the default interface.
For action bars, I use Bartender. Bartender turns all your action bars into movable bars, so you can arrange them how you feel most comfortable. I keep all mine in a box shape at the bottom of my window, so I don't have to go searching for the right spells. It's not the most versatile action bar modifier, but it's pretty nice if you don't want anything too complicated.
Decursive is almost a necessity for healers that use addons. Like Grid, it has little boxes representing each player (and pet) in the raid. When a player is cursed, poisoned, diseased, or has a magic effect, the box will change color and show the duration of the debuff. You can then click on it to dispel it without having to find the player, figure out what kind of spell it is, and then cast.
Clique is a good alternative to Healbot, which I have found to be bloated and unreliable. Clique lets you program in easy keybinds for different abilities. I currently have it set to my most commonly used ones. For instance, shift-left button activates my Power Word: Shield. Useful if you are bad with macros and keybinds, such as myself. (shhh)
Now, I know I am going to get shit for this next one, but it works pretty fabulously for me. Smartbuff. It was recommended to me by my mage friend while I was leveling my priest. You tell it what buffs you can cast that you want it to track, and it will tell you who in your party is missing said buff. You can then just scroll your mouse wheel and cast what's missing. This is pretty great if a few people die in a fight, so you don't have to hunt and peck for who's missing what, or waste another reagent (because we all know they're soooo expensive.)
A must-have for the distracted raider (like healers with tunnelvision) is Power Auras Classic. It gives visual cues (as auras around your character) for buffs, debuffs, etc. It's useful for any class (seals for paladins, aspects for hunters, standing in the stupid, procs, etc.), and works exceptionally well for people like me who usually doesn't have very good situational awareness because we're too damn busy chasing bars.
Three less important, but still nice to have addons are Atlasloot, Skada and Deadly Boss Mods.
Atlasloot, a loot database for all dungeons, raids, reputation rewards, etc., is a pretty sweet little tool to use to find out "hm, where do I need to go for my next upgrade?" I never leave Dalaran without it (especially now that I'm building a shadow set... SHH! Don't judge me!)
Skada is an alternative to Recount, and is much more lightweight and, in my opinion, accurate. It's a better tool for discipline priests than Recount also, because it gives estimated absorbs rather than flat numbers. Take THAT, resto shaman!
Another useful tool for the ADHD raider is Deadly Boss Mods. Mock me if you will, but it's incredibly helpful for people who, again, have tunnel vision. It will warn you with sounds and notices to move when you need to move, stop healing when you need to stop healing, and pretty much anything else you can ever need to know about boss encounters. Some call it a crutch. I call it Raid Ritalin.
I run a few additional addons, but they're mostly fun or vanity things.
I will be updating this with a link to my interface so you can see how these all tie together.