Riff is right, they've stayed true to their sound throughout their career, but their productions do vary quite a bit and for some albums, the production could be off putting.
If I had to recommend a release to someone who wants to get acquainted with the band, I'd actually recommend the 2-CD compilation "XXV". It's a collection of re-recorded songs from earlier Vader albums, from "The Ultimate Incantation" up to "The Beast", so some of their best material, but with a top-notch production. I actually prefer some of the rerecorded versions to the classic versions.
Get the limited edition; it has a very interesting bonus DVD.