Originally posted by Wheatfield The Pentax name on the lens bezel.
A good starting point
I think what adds to a lens' reputation for being great is a certain charactor. An example of a great lens to me is a K55 f/1.8, fantastic build, quick enough and a certain look that belongs only to it. It has charactor, its weak in certain areas for sure but get it in its sweet spot and it delivers a great image with a colour rendition that is unique to it.
The obvious examples of great lenses would be the FA Limited series, they have charactor in abundance, some of the FA*s would spring to mind, the 85 f/1.4 and 200 f/4.0 macro, the former due to being peerless in portraiture and the latter because it is legendary from a pure technical image resolving standpoint but if you look at any image taken with it then you will recognise its charactor straight away, an uber rendition that is just a little beyond any of its peers.
An other lens that has a lot of charactor that springs to mind is the K50 f/1.2, difficult to focus, fairly heavy, but again it has a look unique to itself, it demolishes the other Pentax 50s in my opinion for bokeh, sharpness, the ability to slice our 3D world with the DOF, it is a 'Great' lens.