First, I would highly recommend the DVD "The Miles Davis Story". For about the cost of a CD/SACD, you can learn quite a bit about the man, his works, and the evolution of his music. Regarding five essential recordings ... I agree wholeheartedly with Patrick_odonnell. Much depends on what you like, and the Miles era that most appeals to you.
Miles had a couple of great groups during his "straight ahead" period ... of course the "Kind of Blue" ensemble featuring Cannonball and Coltrane. '58 Sessions (some recorded "live") featuring "Stella by Starlight" is an excellent piece by that same group. "Seven Steps to Heaven" is another date which includes some very good all round cuts.
The second great group with Wayne Shorter, Herbie Hancock, Ron Carter, and Tony Williams put out some great stuff: two of my personal favorites - "The Sorcerer" and "Nefertiti".
Best of the fusion era ... I agree with "In a Silent Way" . In addition to veterans from his earlier core group, two great keyboard players of that period - Zawinul and Chick Corea - appeared together.
Good sound, and happy listening!