Which organism can point us to the evolution of sight?
Which is the least evolved organism that can see?


The term less evolved does not mean anything. Instead one can ask


  Which lineage was the first to evolve the ability to see?


And by "see" of course, I mean detect electromagnetic waves. Seeing does not imply color vision and does not imply accurate vision.

It will be hard to tell really which lineage came up with vision first. But I am sure you'll be interested to know that some cyanobacteria are able to detect electromagnetic waves (reviewed in Nilsson and Colley 2016).