Hard to limit a Miles choice to one so I hope you don't mind a few recommendations. i think you've got to deal with both Miles' acoustic career as well as his initial electric work (until he "retired" in 75). I'm sure you'll get a lot more suggestions for the acoustic Miles (sadly, a lot of folks hated the electric material...but if you're open to it there's some incredibly original, unique and wonderful music here) so i'll also try to give some thoughts on the early electric Miles.
First, if i had to pick only one acoustic disc for you to listen to i guess it would be Kind Of Blue (one of The classic modern jazz recordings). At the same time, all of the classic quintet recordings with John Coltrane are also superb including the 4 Presige records (Cookin; Relaxin, Workin; and Steamin) and records like Round About Midnight. You can't go wrong with any of these.
Also, if you can find a copy of the Miles Davis John Coltrane gig from Stockholm in 1960 (it's been released on different labels and in different formats over the years) this is a steller live performance from start to finish with the best performance of All Blues (one of my faves) that i've ever heard. Coltrane's playing hear is magnificent, freer than on any of the other Miles dates i've heard.
Finally, if you get into this stuff and want even more, Miles' second great Quintet with Wayne Shorter, Tony Williams, Herbie Hancock and Ron Carter also provide a wealth of wonderful material to explore.
On the electric side, i think it's actually tougher and depends on whether you prefer a piano-based sound (in which case i'd go for the remastered In A Silent Way...a beautiful and hypnotic disc) or the more guitar-based heavy funk sound of the later works like Agharta and Pangea. I also highly recommend his Tribute To Jack Johnson (with some great guitar from John Mcglaughin and according to rumour/legend Sonny Sharrock). And, if you're still into being adventurous there's the amazing (and i always thought highly underrated) On The Corner; a record which sounded "ahead of its time" then and in many ways still does.
Live Evil is also one of my favorites and in some ways may be the best overall introduction to the early electric Miles. All of that having been said, it was Bitches Brew that is still considered to be the Revolution (though, personally i've always preferred some of the other electric releases more) and if you're going to check this one out it is worth it to indulge in the complete sessions...the sound quality is much better and you get to hear these tunes both as they were originally released as well as how they were recorded unedited (the same goes for the In A Silent Way Sessions).
Sorry, but i've never been a big Clapton fan...i know, heresy...sorry, but if i'm gonna listen to a blues guitarist i'd rather listen to the the black masters (old and new) and not another white wannabe who makes out like he invented the stuff whose support for the right-wing nationalists in Englend (whose program includes an ugly racist hostility towards "immigrants") is totally irreconcilable with the blues history and tradition that has made clapton what he is (sorry for the digression but i can't help it).
All of that having been said, i do still like Clapton's early "classic" material including the Cream, Derek and the Dominos and Blind Faith -- in no large part due to the other band members. You can't go wrong with any of these.