Just to repeat and agree with what has already been said:
If you want to spend 100-200 bucks, Benchmade's Axis lock is ambidextrous. It is not biased towards righties at all. Move the clip to the other side (the holes are pre-drilled), and you experience the same knife righties do.
Also, lockbacks with a thumbstud on both sides or a hole are ambidextrous. Spyderco makes many ambidextrous lockbacks, several of which are high-end, such as the Manix and Persian. Spyderco also offers make great knives for a lot less than 100 bucks. They aren't top of the line, but they are definitly quality knives. It just depends on what you mean by "top of the line", as a swap-meet-knife fan would consider a Native or Delica to be insane top of the line super knives.
If you want to get something a step or two above Benchmade and Spyderco, the Sebenza is the standard. I cannot say it is the best lefty knife, nor the only, but it is truely the standard. It is also a great value compared to lefty versions of custom folders.