Aloft, Tidy Plates or Name Plate Advanced for nameplates.
PlateBuff for adding your Debuffs to said nameplates.
Icicle tracks enemy CD above their nameplates.
Gladius for enemy frames.
X-Perl or Pitbull for unitframes.
Bartender/Dominoes for action bars.
Parrot or Mikscrollingcombattext for combat text.
Dotimer, fortexorcist or SpellReminder for tracking dots, CD's etc...
Those are my gotta haves I know I lean on addons too much but seriously if Blizzard spent more time and thought on their UI I wouldnt have to use half of them.