How to level sloped concrete ceiling, and is it worth it?
If you want it level, cut furring strips and put up drywall. 

If the precise shape of the ceiling is going to bother you, that might be worth the agony and is a heck of a lot easier than trying to get that much mud onto the ceiling, much less on the ceiling and actually level. I wouldn't bother with it, personally. The slight angle would not offend my sensibilities, and is also better acoustically than a dead-flat ceiling.

For smooth, drywall compound or actual plaster will do. I'd lean towards plaster for concrete, but I'm not sure there's anything to my bias, there, other than my bias. Grab a big ol' knife (or trowel) and go. If using mud, probably the 90-minute setting (aka "hot mix") compound, unless someone else has a better reason than my bias in support of my bias.

Then again, USG (and probably others) has a compound specifically for concrete.


  COVER COAT® Compound is a premium mixture ideally used for filling and
  smoothing above-grade, monolithic interior concrete ceilings and
  columns, embedding paper drywall tape or acting as the first fill coat
  on bead, trim and fasteners. The compound dries white for a quick,
  easy finish.


Further reading:

Gypsum Plaster over concrete  and Portland cement "plaster" (stucco) over concrete