Besides putting in the good stuff, have a variety of good stuff. This way, you can show your versatility.
If you edit films, perhaps you can produce trailers and use them as portfolio. Or if you had edited a very good sequence (e.g. Hollywood car-chases, Hong Kong Kungfu fight scene, etc), just cut out that the entire scene and use it as a portfolio clip (the scene shld have a proper start and end).
I feel that there are many ways to present a portfolio, but the general guidelines should be: 1)each showcase should be short and serves a purpose, 2) show off your versatility and capability, 3) show what makes you different.