Trouble is my favorite doom metal band for exactly the reasons shown above, so props to the above posts there. The riff at about 2:09 in Revelation (Life or Death) is my personal favorite from them. The thing with Trouble is that they had no issue writing anything from what were almost thrash riffs down to some really slow dirgy (for its time) ones; they understood how to convey a range of moods and emotions in sheer riffing.
And yeah, I had actually noticed that my copy of Psalm 9 (the 1994 CD re-release) sounded a bit off at points, and now listen to some cassette rip I found on a blog which improved things. Should probably try and find a quality vinyl rip one day.
Once "atmospheric" and "symphonic" and etc black metal started to take off, riffs were reduced to little more than note progressions on which to apply keyboards or a couple of jangly chords repeated over and over. Sure, albums like DMDS have plenty of great riffs, and if we include first-wave stuff fundamentally built on speed/thrash riffs then you can find good stuff there too.