I don't really have a preference for a certain type of solo. But in terms of riffs, I like dark and/or emotive riffs. I like brutal/technical dm, but I don't like riffs that are brutal/technical just for the sake of being brutal/technical (modern Cali death, anyone?). As for fast or slow, it depends what mood I'm in. These bands write my favorite riffs: Mourning Beloveth, Arghoslent, Sacramentum, The Chasm, Amorphis (Karelian Isthums, I hate the new stuff), Nile, Suffocation, Dissection (not Reinkaos, though), Dawn, Demilich, and a few more. As you can see, these bands cover a variety of styles.