The one you are least likely to see is the one coming straight towards you, but again that's the one which is most likely to miss you - what are the changes of being in the exact opposite heading at precisely the same height? I have never heard of this happening in any accident.
David,
It's not just opposite heading - it's a constant relative bearing that leads to a collision and because the target is not moving left or right, it is more difficult to detect.
Having said that, the "big sky" theory helps a lot as does flying at non-standard altitudes and avoiding choke-points like VORs and VFR routes near the edges of CAS.