The readers of Total Guitar and Guitar World magazines voted Jimmy Page's riff from Led Zeppelin's "Whola Lotta Love" as the greatest guitar riff of all time. 

We can read in Total Guitar's July 2021 issue the following description: 

"In 1969, the year Neil Armstrong first set foot on the moon, Jimmy Page launched his own giant leap for mankind. 'Whole Lotta Love''s guitar figure took just 2.7 seconds to play, but it immediately projected music into another decade. While everyone else was still playing the '60s, Zeppelin were now playing the '70s. It wasn't the first great riff, but it is the defining one. It's why riffs became central to guitar music, the reason bands search for the guitar hook that can propel a whole song — or even a whole career".

So the top ten list is as follows:

01. Whole Lotta Love (Led Zeppelin)
02. Crazy Train (Ozzy Osbourne)
03. Back In Black (AC/DC)
04. Smoke On The Water (Deep Purple)

05. Ain't Talkin' 'Bout Love (Van Halen)
06. Enter Sandman (Metallica)
07. Iron Man (Black Sabbath)
08. Walk (Pantera)
09. La Grange (ZZ Top)
10. Purple Haze (Jimi Hendrix)

The list of the greatest riffs of all time features also "Smells Like Teen Spirit" (Nirvana), "Killing in the Name of" (Rage Against the Machine), "Psychosocial" (Slipknot) etc.