Hard to pin the Beatles down to one song, since they evolved so much in their 8 year run of brilliance. Plus, they should get one song each for Paul and John. So I'd add Love Me Do, just to represent John & the early period. Plus I think the lyrics are brilliant.
I like Black Dog better as a signature song. Zeppelin was all about stealing blues riffs, and that one even got 'em sued!
OK, hang on, did you not say signature SONGS? These are ALBUMS! What's that about? At least choose one track as a signature. Besides which, I'm going to throw out Bike as the signature song of Syd Barrett-era Floyd, with Money probably best representing post-Barrett.
I have to agree this was his signature tune until he went all disco on us. Now, of course, it would be "Night and Day" or some old jazz chestnut like that.... :-)
This surprises me a little. I guess I think of Rumors as such a definitive album for Fleetwood Mac, I would have expected one of the songs from there to get the nod. Personally, I absolutely love "Bleed to Love Her", but that really only represents Lindsay, not the whole band.
As much as I'd love to mention any number of more interesting Who songs, I think the 2 choices above work best. As for Skynyrd, ditto.