right now i'm into blackwater park because my friends and i are going to cover it. i got this kid to do vocals that sounds almost exactly like akerfeldt, its cool. anyways, i love the riff about 1:50 into blackwater park, its a real solid, kick ass riff. i really dig the interlude, its so calm. also. the riff that swells out after the interlude is awesome.
some other riffs i like are the beginning of the night and the silent water, the riff in serenity painted death with the harmonics (its got a cool jazzy groove to it), the riff played in demon of the fall underneath the line "demon of the fall", and the beginning to the apostle in triumph is one of my favorite opeth acoustic moments.