Which Enslaved cd should I buy?

Monumension is my personal favorite, and I recommend it highly. Isa is also incredibly good, and Below the Lights and Ruun are quite excellent, as well (maybe just a notch lower). You really can't go wrong with any of those four. Their earlier stuff is interesting, but much of it is not in English, and, in general, it sticks a little closer to genre conventions. 793 off Eld is a classic track... to be honest, their whole discog is pretty fucking solid.
 
After listening to Enslaved so many times in the past year, I'd say Monumension has caught my attention the most, with Below The Lights closely behind.

Honestly though, just get any of them except perhaps Frost, because they are all good. (Frost is okay... but for an Enslaved release it really doesn't cut it.)

edit: hmm fsr I said month instead of year, oops
 
start with Below The Lights, Isa or Ruun, the most accesible. and whatever you do, don't listen to hibernal dream.
 
depends what era you want, of course. if you want a good introduction to current era enslaved then go monumension or below the lights (the latter probably easier to get into for a new listener). if you want to get into the older material then start with frost, eld or bloodhemn