I'm big into older (90's) Scandinavian metal so I am a little biased on where I choose my riffs..although most listed above are very good.
In Flames - Every single song from their Subterranean EP. Actually most In Flames records from Subterranean to Clayman are chocked full of songs with great rif***e.
https://www.youtube.com/watch?v=UJ2pT6mBrTM
At The Gates - Slaughter Of the Soul and The Head of the Hydra
https://www.youtube.com/watch?v=QhLqkPtp-Hg
Edge of Sanity - Blood Colored (also albums Spectral Sorrows/Purgatory Afterglow/Crimson)
https://www.youtube.com/watch?v=3emFczewdvI
Opeth - The Night and the Silent Water (first two albums have a bunch of good riffs)
https://www.youtube.com/watch?v=U9RzeEwXb_c