Must haves:
Carbonite for questing.
Quartz for custom cast bar
Elk Buff bars for tidier/smaller and more informative buff/debuff views
ClassTimer for tracking cc/offensive timed skills such as snares, dots.
Skinner for a "cleaner" background texture on panels
I ran with many mods but the above I found pretty critical. I used PitBull for unitframes, Squeenix for my minimap, AutoBar for consumables, Bartender for bars etc etc etc.