It could make a million for you overnight
Having written my review of Murmur (below) I was taken to thinking of other influential musical artefacts and came to the conclusion that the Beatles' single Paperback Writer / Rain is quite possibly the most important single ever released. Let me explain why:
Released between Rubber Soul and Revolver this single marked the start of the Beatles ascent into psychedelia. All of their most progressive and inventive music would be created after this point.
This single, particularly the flip side Rain, marked the real beginning of musical experimentation. Although hinted at earlier, the feedback at the start of I Feel Fine is the first know occasion of recorded feedback on a record and the Sitar on Norwegian Wood being another first that would later influence The Rolling Stones' Brian Jones greatly, it is this record that has two major 'firsts'. Rain is the first ever record to feature a tape-loop and is also the first use of having tape played backwards on a record, two techniques which have become common in music today (try and imagine Missy Elliot's Work It without the backwards bits).
The second major 'first' is the two music videos that were made to accompany the two songs. Although the Beatles had made promo clips earlier these were mainly performative or weren't made specially to promote the single (the films A Hard Days Night and Help could be seen to be a series of music videos strung together by a thread of a narrative). The clips for Rain and Paperback Writer are in colour and are not just performance pieces.
Aside from this is the obvious influence of the sound of the record. Listen to Rain and you can immediately see where the Oasis 'sound' came from; Lennon sneering and snarling the lyrics decades before Liam made this type of delivery his trade-mark. The drumming in this song is perhaps, along with Strawberry Fields, Ringo's finest and most complicated drum pattern. McCartney's bass floats throughout the piece and is one of his most distinctive and interesting bass parts. The two guitars chime throughout.
It is not the greatest single the Beatles ever released but it does show the band at their tightest (all their instruments play distinctive, different and yet complimentary parts that constantly sound like the songs could tear themselves apart at any time but are never allowed to. In the instrumentation of Rain you can hear the Beatles starting to go their separate ways and break out of the cosy 'Fab Four' image that they had stuck to for half a decade). Whether it is the most influential single is a matter for conjecture but I can't think of anything that fits the spot better. What does everybody else think?