[rawr]

Samyang 14/2.8 is really proven

And has good software support - including a very effective DxO PhotoLab lens module for Pentax, and a Lightroom lens profile too.

[Sandy Hancock]

If you want all of these:

Wide prime or normal zoom with wide end (17 or less)
Fast aperture (at wide end)
No coma at wide end and wide open (for stars shooting)
Autofocus (required)
Very sharp at wide end

Your definition of this:

Reasonable price

needs reassessing.

The DA* 11-18 is your best option.

I have the DA 11-18, and I have the Irix "Blackstone" 11. I find that both are superb rectilinear lenses; the fixed focal length of the Irix is useful in some situations; I find that its only deficiency is that it's manual focus only, though it also lacks the flexibility of the zoom lens.

The big difference is the price. The Irix lens is built like a tank and works great, at half or less-than-half the price. I reckon the Irix 15mm would be as good as my 11mm version. And if you want it cheaper, still (at the expense of weather resistance), get the "Firefly" version (also lighter, being constructed mainly of plastic).

[Sandy Hancock]

Autofocus was one of the OP's must-haves.

If that's negotiable, the Laowa 11/2.8 Zero-D is also a contender, but also not cheap.

Good point, @Sandy Hancock. However, I'd argue that autofocus isn't much of a bonus on lenses with that wide an angle of view, since almost everything will be in focus anyway. You actually have to decide to make things blurry and use the widest aperture and focus for close up to blur things up.

And as to the price, I like to compare things like lenses to things like milk and gasoline. You don't have to replace a lens every week, so the increased cost of one lens over another isn't really all that dramatic when amortized over time, while five cents a gallon difference in the price of gas will add up. I worry a lot more about finding the best gas station than I do finding the best value in a lens; I'm willing to pay more for the quality regardless of dollars per square millimeter. (When I first started driving, gasoline was $0.19/gallon. Can you imagine how much I've spent since then on the increased price of gas?)

[Sandy Hancock]

almost everything will be in focus anyway. You actually have to decide to make things blurry and use the widest aperture and focus for close up to blur things up.

Even with the depth of field inherent in ultra-wide lenses, critical focus can be quite important if you really want a certain element of the composition to pop.

[photogem]

What about the older lens, the Pentax DA 16-45 F4? Is it still worth something on a modern K-S2 compared to a DAL 18-50 WR RE Kit and a Tamron 18-250 superzoom? I don't have interest in WR, but I do have interest in both wider angle of view and sharpness.

The DA16-45/f4 is a great lens if in good condition.
Some which had been heavy used or even worse, hit a bit when the barrel was out, can have pretty bad wobble. Then results are not good at all!

But if in good condition it is a great lens.

I remember speaking on the phone to one of those older Pentax lens repairers in the US. Forgot his name but he was well known and has worked for Pentax for ages. He told me that the DA 16-45 is a kind of floated design similar to the FA31. I cannot comment on this myself.
My excellent sample workes great on the KP.

Another sample without wobble but a tiny scratch is always in my bag with a K5 in my car.

Has also nice close-up ability!

[Bertrand3000]

I nearly got a DA 16-45. But even if it looked in great shape, front and back element clean with absolutely no dust, the lens was not recognized by the camera, displaying F--.

On closer inspection, I noticed that the aperture level was bend, it did not return to its original position when pulled. Also, there was a loose element inside the lens, sometimes blocking the zoom ring. There was a suspicious hole in the mount as well (perhaps the A-pin missing).

It baffle me to see how people can damage costly equipment like this. It does not look like fall damage.

Too bad. That's why I don't like buying used.