That's a toughy because I'm super biased - met the Opeth guys in May and they were super cool. I love Blackwater Park and My Arms Your Hearse....but....I cannot sing along to them, doesn't hit me that way.
Death I've been listening to for years and years, sing along to (well try) and just know better. It's definately catchy stuff, so I'd pick DEATH.
These bands are super different from each other, so it was hard to choose.