I agree with Edge Of Sanity and ND the easiest band to understand would probably be the first Suffocation album though ...J/K.
I think the fact is, if you are new to the whole dm vocal thing, it will all be hard to understand. The albums you listen to the most and follow along with the words will probably be the easiest to understand.
For example, Tomb of The Mutilated, I listened to that shit over and over for like 3 years when it came out, besides being memorable, and reading the words, I think i just remembered all the words.Looking back, it seems as if the words are easy to understand but only because I knew the words. Not as good of an example, but I know every word I sing, and have no trouble hearing the words, but when I record the songs, I write lyrics for months, but arrange the the lyrics to the songs last, (sometimes the night before) if you asked me what I sang the day after recording, I would have no clue.
There are exceptions I suppose, but either way you put it, some dude, is screaming words and cramming words to fit over time in music. Naturally it will be hard to understand.