Which song - if any - would you say was the least typical of the band's repertoire?
Over the years - and certainly in the second half of their career - the band experimented with a host of different styles, some of which were more successful than others, depending on your point of view. For example, on The Lamb there was The Waiting Room - an atonal jam which was the closest that Genesis ever came to the sort of "free jazz" as performed by the likes of Ornette Coleman or Miles Davis during the early seventies - and on A Trick Of The Tail and Wind And Wuthering Phil pushed the band into Weather Report territory with Los Endos and Wot Gorilla respectively.
Come the eighties and they really threw out the rule book. Misunderstanding tapped into a more American sound, specifically recalling Toto's Hold The Line. On Abacab, they killed as many sacred cows as they could find, abandoning musical traditions like big chords, complex instrumental passages and a tambourine on the chorus! In their place was a stripped down sound with an almost punkish attitude (indeed, Who Dunnit actively invites the comparison with punk) while influences ranged from reggae to r'n'b to the blues to a kind of industrial funk on Dodo.
For me, the one song that really stands out, though, is I Can't Dance. The moment that guitar riff starts, you'd be hard pressed to think it was Genesis. And that is what I suppose I'm getting at: what song of Genesis's would you present to someone who didn't normally like their stuff with the proposition of "Guess who this is?"