These are addons that are suggested for making life easier while clearing veteran content, as well as raid content. Addons highlighted in red are ones that you absolutely must have for raiding, because the advantage of having them is ridiculous.
Bijiplates - Updates unit health bars, replaces current system.
BetterPartyFrames - Changes party frames, making it easier to see health and shield.
ThreatMeter - So you don't pull off Donatist in the first 3 seconds of X89.
Groupradar - Shows you where your group members are.
ENTspy - Pretty much a map hack. This addon is absolutely amazing and you should never be without it.
TapThat - So you don't have to do annoying quicktime events.
SCastBar - Shows your spell cast times.
Aura Mastery - Tracks buffs. Required for raid.
NotNowMasterLoot - Hides the huge frame that master loot creates in a fight.