The simple answer is only the ones that improve your performance.
o.O .....and.....which ones are those?
This is a much tougher question. I've been in and seen many guilds that require certain add-ons. Does simply having recount or omen or DBM make you a better raider?
Omen is a bit of non-issue now a days. You saw this one more in the old days when it was easy to strip threat off of a tank.
See the thing is simply downloading these add-ons is not beneficial for most raiders. For one thing omen should you the entire groups threat. It was difficult to parse through and find yourself on the list and even if you did you had to take a few milliseconds or more to parse this data.
THAT IS NOT HELPFUL!
DBM suffers the same fate, out of the box it simply provides both too much information and too little. Does a non-tank really need to know how many stacks of Tank Death debuff are being applied? No. Do you really need a warning to tell you not to stand in the big red circle? No.
DBM can be extremely useful but only if you go into it and turn off the things that never apply to you and turn on the ones that do. After a wipe ask yourself which warnings you ignored and which you spent time parsing the multitude of bars to pay attention to. Turn off the ones you ignored, check for ones that track things that you spent time wishing you had a warning for and then when a bar appears you know that that bar is special to you.
Recount/Skada are a mess in the hands of the ill informed. First off do you really know what they do? How about what they don't do?
What these add-ons do is track the amount of damage you put out and the length of time you are in combat mode. They don't care what target you hit, if died, if you stood in the stupid the whole fight, if you front ended your damage then picked your nose while your mana bar slowly crept back up. The HPS charts are even worse, they are almost completely useless as a method of evaluating healing.
Finally if you just run an add-on without thought or care as to what it does you may never see the things the add-on is supposed to show you. It does no good if your DBM warning are at the top of your screen and you never look there during a fight.
This is one of the reason I love Weakauras, when you setup an aura you are invested in its creation and its success. You have ownership of it and its yours. I guarantee the WA icon you spent 20 minutes making is going right where you can see it every single time its up. If only because we humans are just vain like that.
At the same time I absolutely hate when people say "I just use the default UI. Blizzard designed it and so it should be all you need."
You know what else Blizzard designed? The ability for creative people to make add-ons and further the ability for players to customize their UIs to insane degrees.
Few other games allow this much modification of the UI.
If that doesn't convince you then try this one on for size:
The default UI is designed to work for new players and max level players, for raiders and pvpers, for healers and tanks and DPS, for 11 classes and 34 specs. It is meant to work for Granny and for the young-uns.
The default UI is over generalized to the point of weakness. The only answer I would accept for someone not tweaking their UI is "It works and I'm too lazy to optimize it". Fair enough.
The rest of us should work to increase the amount of useful information and the efficiency with which it is displayed while removing extraneous noise and poor placement. Any add-on that is distracting you is making you a worse raider. Any information that takes time for you to find, parse, and react to is inefficient.
Don't just shrug it off as part of the add-on that you are gonna have to live with, find a solution. As someone who records many of my raid attempts I have found upon re-watching my own play that something on my screen was completely ignored (I removed it) or something was out of the way (I moved it) or confusing (I made it bigger and moved it to a place that made more sense to me). I'm constantly looking for what I'm doing poorly and working to find a solution.
So my list of required add-ons for raiders is not filled with something you can find on curse or wowinterface, instead it is the idea that what ever you use make sure it works for you. Make sure it helps you and does not do more harm than good. Good raiders shouldn't have to be told what add-ons they must use because a good raider is a raider who is constantly and perpetually looking to improve her raid performance. She will find the add-ons that work....or she will find herself on the bench wondering why.