I think a great many of Miles' albums get overlooked, simply because Kind Of Blue somehow got stuck on page 1 of "Jazz For Newbs." Nothing wrong with that but it's just a good example of Miles, not the be-all, end-all.
If I just want to chill with Miles, the Cookin'/Workin'/Relaxin/Steamin' albums still really do it for me. Recommended.
But if I have to vote, it's Birth Of The Cool all the way - it really announced the arrival of Miles but (even moreso) it's musical content was so hip that it helped to permanently morph the art form itself.
Bitches Brew was also significant, but in a somewhat opposite fashion, announcing his departure from everything he had done before. Just a few years before his death, miles was interviewed and asked why he didn't play any of the straight ahead stuff he played for 20 years. His response was "man, that's boring sh|t." I report, you decide...
If you like Kind Of Blue, I recommend Cannonball Adderley's "Something Else" and if you get into Birth Of The Cool, check out Gerry Mulligan's "Re-Birth Of The Cool," a total remake of the original, 40 years later.
I've said it before and will again - Miles and Trane, while good examples, only scratch the surface of jazz. There are endless paths to explore from there.