To me, the best of the best must have a certain *feel* to them.... that certain indescribable quality. This does not always mean they are necessarily the sharpest, have the best bokeh, etc. however.
For me, the best of the best really depends on the category of photography also.
So...
for
macro photography, the best of the best in Pentax land IMO is the.....
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PENTAX M 50mm F/1.7 Im convinced its the sharpest lens I own for macro work bar none.
LARGE photo:
http://farm2.static.flickr.com/1255/5104954877_cbe58dfaa8_o.jpg
for portrait photography, the best in my arsenal is the.....
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The wonderful
Pentax M 85mm F/2. I've already expressed my love for this lens enough, so i'll spare all of you
What I will say is that it has something all of my other lenses do not.... I will call it pixie dust for lack of a better word
Overall though, the best of the best Pentax lens I own would have to be the
K 50mm F/1.2. It is by far my most versatile lens, and is the first I grab when going out, no matter the occasion. If im just planning on hanging with friends I'll bring it, if its portraits well F/1.2 does make things interesting, if its landscapes this lens is about as sharp as they come when stopped down, etc. etc.
Honorable mention:
#1 - F 135mm F/2.8: An amazing lens, with stupidly fast AF and fantastic image quality. I have not used it enough however to call it my best of the best
the rest are all *ok* for various personal and/or technical reasons