Well definitely start with the first self titled one.Kind of Slayerish/Panteraish.They are also big Testament fans.The second and third albums ( The Haunted Made Me Do it ) and ( One Kill Wonder )are very thrashy with Death metal kind of vocals.The music is relentless with lots of changes.The fourth album (Revolver) has the original singer back.and the music is thrashy but also a little more hard core.After that things kind of fall apart for me.The Dead Eye)Still with the original singer but everything is slower and more melodic,less and shorter solos.Still kind of cool.But on the low end for me.This style continues for the next album (Versus).Maybe even gets weaker.There are a couple cool songs here and there(Moronic Collossuss). The next album I didn't bother getting.I skimmed it.And it really just seemed like dog crap to me.And I was done supporting that kind of direction.
The album coming out sees them back with the second singer the original drummer and a new lead.I am excited for this one.and it will have a guest by Chuck on it!
Of course this all just my opinion.I'm sure there are people who like the last 3 albums.
Oh! if you do get the albums,try and get the ones with bonus tracks.They are good.