Stigmata for me. All three albums though are criminally overlooked.
Are they though? Arch Enemy is generally one of the first half-dozen or so bands brought up when melodeath comes up, and the first three often seem regarded as the best. I guess it depends on what crowds you're around. I agree that they're great, but there are dozens and dozens of other bands who don't much attention at all.
I agree they are one of the most successful melodeath band....but didn't they become so after Liiva left and Angela Gossow came on board? It seems to me they became much more of a "phenomenon" (relatively speaking, of course) at that point, while before they were very much a niche band (with "famous" members, sure, but still within the metal scene). My guess is if you ask fans now, they probably don't know much about the first three albums, but I might be wrong...