Cost wise I couldn't go past the Pentax DA 35mm f2.4. Not quite as good as the Pentax DA 35mm F2.8 Macro ($700) but a great lens for $300.
I got mine from B&H who where selling it for $179US... bargain I thought.
I considered a lot of lenses for my K-5... knowing what I would want to shoot helped define where I would spend my money... for me this means Wildlife, Birds and general photography bring up the rear.
So a Pentax 50-135mm f2.8 and a Sigma 120-400mm Super Zoom (haven't ordered yet, waiting to see if Pentax announce anything new soon) for the first 2 uses.
The 35mm f2.4 brought up the rear for portraits & scenic shots. I may consider a 12-18mm lens later ...
I looked at the Pentax 18-135mm WR but didn't like what I read in the reviews... even with the K-5 Inbody lens correction that lenses sounded a little to distorted at either end (just my 2cents).
So that's my advice... decide what type of shots you really want to take and target your lenses accordingly. This site has great user reviews on lenses.. so does
All Tests / Reviews that site.