Probably Wreath. When I listen to Opeth, I tend to forget what kind of music it is because I just say to myself "its Opeth thats all", but when you listen to a song like Wreath, objectivly, it really is a fuckin good death metal song, especially the intro riff and a few others.
This is also true about a lot of other songs. For example, I really don't think of Opeth as a black metal band but some riffs are sometimes very black metallish. And I love how in the same song, you can have black metal moments, death metal moments and very clean prog moments also. Thats why I think Opeth is so rich... musically