All depends what your using the microphone for. If you are using a shotgun for dialogue you will want as directional as possible to give clean dialogue and block out room noise. Most shotgun microphones will be bi-directional (record front and back). They will be very pinpoint on the subject. What you need to watch out for with these microphones are the backs. Make sure the back of microphone is not picking up any room noise (fridge, people talking at table, etc.)
If your using a lavalier mic, then you will want omni directional to pick up the subjects dialogue since it will normally be placed on shirt below mouth.
I'm looking for a microphone that only record dialogue from which ever direction you were pointing to.......not picking up noise from the back or noise from outside
Depends. Many pro audio folk prefer a hypercardioid over a shotgun for dialog..as long as they can get the mic reasonably close to the subject. The advantage is less chance of voice going off-axis as subject moves head and less rear pickup which can be very helpful in a room with reflective surfaces. It's a bit counterintuitive in that you would think higher directionality would prevent pickup of echos..not the case if the mic has a significant rear pickup. For the sake of provideing the exception, I believe Sanken makes a pricey shotgun mic that has minimal rear pickup.