It's hard to say which lens is the sharpest. But from photodo tests, the F50/2.8 macro got the highest MTF among all Pentax lenses tested there.
As to make background blury, it depends on the subject distance, background distance, focal length and aperture used. In general to shoot a closer distance target with a longer focal lens would give your blurier background. Normally a 50mm or longer focal length would be good enough to isolate background from the main subject if it's within 1 to 1.5m distance.
This would be a bad way to approach lens selection and understanding.
Not to mention that every one has their own preference/poison and you'd end up with too many choices and suggestions.
I suggest you do some read up on the basics of photography (eg. shutter speed, ISO, aperture )
YouTube also has a good resource "Photography one on one" from Adorama, so don't even need to read
Fengwei gave some good advice above on how to get less depth of field to make the background blur.
To me most of the Pentax primes (old and new) are generally sharp (or rather sharp enough). No need to be sharpest.
All the DA* lenses are sharp as well.
I've only encountered really not so sharp lenses with a Sigma super zoom and a couple of odd ball no name old lenses (maybe they were just bad copies :think
Choose your lenses based on angle of view and intended subject.
If you got no money to fork out for fast, large aperture lenses for subject isolation and blur backgrounds with your images, then consider these software options.
Trust me they work.