These are my answers to suit my needs. There isn't a "best" answer for all users unless all users have identical needs. Take them as you will.
I like to buy, try and sell lenses, so I currently own or have owned at some time the following normal lenses, beginning in 1977.
K50/1.4, Super-Tak 50/1.4, SMC Tak 50/1.4, M50/1.4, M50/1.7, FA50/1.4, A50/1.4, K50/1.2, M50/2.0, SMC Tak 55/1.8, K55/1.8, Auto-Tak 55/1.8
Generally speaking, across the range, ALL Pentax normal lenses produce excellent reuslts in general photography. They excel at certain specific tasks, which vary by maximum aperture. These are my observations:
- Pentax f/1.7 normal lenses were instentionally designed to offer a comparatively flat field and sharpness across the field, and to be used for flat work (photographing text subjects on a copy stand when that was how printing negatives were made).
- Pentax f/1.4 normal lenses were intentionally designed to offer more pleasing bokeh than f/1.7's. As a consequence they tend to lose sharpness at the corners at large apertures.
- Pentax f/1.2 lenses were intentionally designed to be the penultimate normal lenses, sharp at almost any aperture, with potentially the finest bokeh simultaneously.
- Pentax f/1.8 lenses were intentionally designed as a great compromise between flat field and pleasing bokeh.
- Pentax f/2.0 lenses are the smae optical formula as the f/1.4's (I thnk) [EDIT: Axl in the post following corrects this to f/1.7] but were intentionally crippled by adding internal baffles so that maximum aperture is limited to f/2.0. They were marketed as budget lenses, but they still produce great images at middle and smaller apertures.
Larger max-aperture lenses are easier to focus in low light and can capture a sharp image with less ambient light than their smaller max brethren, if that is important to you.
Optical sharpness distinctions between these lenses are rarely apparent to me when producing prints in sizes smaller than 8x10. By f/8, sharpness distinctions between these lenses largely disappear altogehter. For those of us who don't make a living selling gallery-sized prints of our photographs the lens choice is somwewhat irrelevant.
The big differences are aesthetic - and subjective.
The coatings formula used on these lenses changed over time, so that color rendering from one optical-formula-identical lens series to the next has also changed (Super Tak >> SMC Tak >> K >> M - and even within the series), and color rendering may become a decisive factor.
The construction philosophy and materials evolved over time, so that Takumar lenses are pleasing to use to some, but overly heavy to others. "A" lenses, though having the electrical contacts to make metering efficient in newer cameras, may have plastic components that aesthetes (like me) don't care for.
There really isn't a right answer other than the answer that suits your needs and desires.
Personally I enjoy using K cameras and Takumar and K lenses because I like the way the experience of
taking a photograph feels and sounds with that equipment. Sometimes M cameras and lenses are small enough that my large hands and fingers aren't comfortable with them. If I lose some tiny bit of photographic perfection (which I doubt) by making K film and manual-metered digital my choice, that's my compromise.
For my K10D (soon enough to become a K-5) I use the FA50/1.4 for my AF normal lens (we'll forgo that debate over whether the FA35/2 is the "New Normal" for a crop-sensor DSLR), so I really don't need the A50/1.4, but I would buy an FA50/1.7 if I came across one).