What's your favourite 20 seconds of music?
The start of 'Jessica' by the Allman Brothers ...... it means it's 'Top Gear' time again!
Tubular Bells.
Plaistow patricia by ian dury, makes me laugh everytime. 
Last few seconds on 'And You and I' from Yes. Hearing Steve Howe on it's guitar is causing goose bumps for sure.
opening seconds from 'Unfinished Sympathy' from Massive Attack. Great 'hifi-set-checker' 
The riff where the priests show their anger at Alex Lifeson's "discovery" of the guitar, followed by Neil Peart rattling down his long row of Tom Toms at breakneck speed. Rush 2112.
O Fortuna - http://www.youtube.com/watch?v=GD3VsesSBsw 0:05~0:25
Insomnia - http://www.youtube.com/watch?v=LuqEbRzy_t8 2:18~2:38
40 seconds of music I could listen to over and over.
The live version of Hawkwind's "You Shouldn't Do That" 4.49 to 5.17 (OK slightly more than 20 secs). The melody of Lemmy's bassline with Dave Brock's sensational power chord breaking in at 5.06 leave me even now with the hairs standing up on the back of my neck. Wish I'd been old enough to have seen them in '72! Maybe not the most hi-fidelity of music but (imo) wonderful all the same.
The intro to Move It by Cliff Richard - the only real British rock'n'roll record of the 50s
The first 20 seconds of the Beatles I feel fine does it for me .
Fleetwood Mac and 'the Chain' intro for F1 racing . . . and what about Albatross, choose any 20 second snippet to drift along.
How about the introduction to J S Bach's Toccata and Fugue . . . big system or big speakers, big volume, feel the hairs stand up!!!! Cheating a bit 1 minute 20 seconds, let it run for a further 2 minutes . . . superb music. Listening to it now 11.20am . . . phew, a bit like having a double whisky before 12 noon?
CJSF
Guns & Roses
Sweet child of mine those 30/40 sec gitar intro put my hair up allover!
'Ride of the Valkyries' at the beginning !!!!!
The guitar riff on The Smiths - Handsome Devil
even Johnny Marr can't play it as good!
The last 20 seconds of Tha by Aphex Twin