My favourite kind of riff is a well-presented midriff, preferably on display in summer...   But seriously, arguably the most iconic riff in rock music is the one that insistently drives "(I Can't Get No) Satisfaction" by the Stones. By the way, wiki also describes it as a hook. The long feedback note at the start of "I Feel Fine" was initially an accident caused by Lennon getting his guitar too upclose and personal with an amplifier, whereafter George Martin suggested incorporating it in the song. The guitar passage following it is a great example of a riff.