Gershwin's "Summertime" is truly the best song I've heard my entire life!!!!!
I mostly listen to 60's/70's music and it's not really riff-oriented, so it's hard for me to pick a riff as such. Bill Withers' "Ain't no sunshine" comes in right after "Summertime", but it hasn't got a riff as such.
Maybe I should pick some metal instead:
Guns N' Roses "Nightrain" - my favorite "metal" song indeed, but haven't really got a riff as such either!
Burzum "Det som en gang var" - The people who know and like this song probably know what riff I'm talking about.
Opeth "Dirge for November" - The lead guitar riff after the distorted guitars come in.
Opeth "Bleak" - the riff under the "Devious movement in your eyes" part.
Iron Maiden "Rime of the ancient mariner" - the riff after the narration, the "The mariner he wished he died" part and the drum fill.
Oh yeah, there's also a funk riff that I think is among the best shit I've heard; P-Funk All-Stars' "Give up the funk (tear the roof off the sucker) live-version from "Live at the Beverly theater in Hollywood". The riff I'm thinking of is originally from Bootsy's Rubber Band's "Bootzilla" (Bootsy were a part of the P-Funk mob which included a lot of people which is too many to list here (over 70)) and it comes when someone whistles. Probably the best riff I've heard in funk!