Anthems seems to be a popular choice, that I often go with. Emperor, being my favorite band along wiht Opeth, I have a hard time choosing a favorite album. The Emperor/Enslaved split shows their incredible and epic songwriting ability while still attaining a fair amount of rawness. Wrath of the Tyrants is really for the most part pretty bad, unless you are looking for some very true black metal stuff, which can be pleasing at times.
Nightside improves on the production from the split with Enslaved and the music is taken many levels higher in terms of atmosphere and emotion.
Anthems really shows their technical proficiency and progressive nature. The music is much more folk based in some cases and relies more on typical classical composition.
I am one of the rare few that quite often argues that IX Equilibrium is possibly my favorite. Though it lacks the coherancy as a whole you can basically imagine it as a mix between Anthems and Prometheus, with the atmosphere and more black metal nature of Anthems and the progressive nature of Prometheus. The songs display vast and chaotic compositions that hit you like a massive wall of sound.
Prometheus is the most progressive, and rightfully so, Ihsahn wrote it with the fact that he wouldn't have to play it live. Often 3+ guitars going on at once with bass and multilayered keyboard parts. Composition is a bit lacking in that the songs are often basically split in half, each half identical to the other almost.
You really can't go wrong with Anthems or IX in the beginning...