I personally tried some of the different options in (or around) the focal range the OP is interested in.
The word "tried" is carefully selected cause this is what I've done with some lenses. I didn't use them for a time long enough, and in different enough conditions, to have proper opinions. Just impressions.
There are FOUR (maybe five) versions of the Sigma 2.8/24-70mm AF, from the old Aspherical to the newest HSM IF.
I own what I believe is the penultimate version, the EX DG with HSM motor.
It's a great zoom. I have used it for long enough and I am sincerely impressed.
I prefer primes, and most of the times I use MF lenses, unless AF is absolutely required. Though this specific lens has been used many times, and I can say it's one of the best zooms I ever tried.
I shot under different conditions, wide open and stopped down. I guess (educated guess, not proven in any way by hard facts) that this specific version of the Sigma plays in the same league of the new D FA. I expect the Pentax/Tamron to be better, though at a certain level differences become quite subtle. Even flare is tamed - at Pentax level I'd say - better than with older Sigma's.
I don't know if the IF version is really that much better. In theory it should, at least at close range.
The Pentax-FA 24-90mm is not a bad lens considering the range. I didn't use it enough to expose its weaknesses, stopped down should be fine even on a high resolution full frame sensor. Vignetting and distortion can be fixed using the right profile in Lightroom.
If 24mm is not needed, there are a number of zooms that could satisfy most users, especially those who are fine with primes, and need AF only in specific circumstances.
When I used the Sigma EX DF (film times, successor to the Aspherical, and before the EX DG) I was quite skeptical at first.
It gave punchy contrasty images but it was very prone to vignetting, especially at the wide end.
Later on I brought it to the local repair center ("Sigma-certified") to be cleaned. I had used it in a very dusty environment and it really needed a good cleaning. I discovered that one of the sleeves, either the zoom or focus one, was slightly bent. Once the part was replaced and the objective cleaned the overall performance got much better. It still vignettes, but not in a disturbing way. Leaving out aberrations, this Sigma gives saturated, contrasty images, and even flare is rather well controlled. If you find one it should go for a very decent price, making it an interesting option, with a great price/performance ratio.
Recently I acquired a battered Tokina AT-X Pro 280 2.8/28-80mm. The build is great, as most Tokina AF lenses. I'd rank them No.1 in build quality. My example has crashed badly at some time during its life. The filter ring at the front is broken at the two sides, and manual focusing is not smooth at all. Despite the accident it sustained it's actually quite sharp. The main problems are lack of "punch" and rather muddy colours. It is possible to improve the pictures in PP, though even after PP they are not at the same level of a straight conversion from the Pentax-FA 28-105mm f/3.2-4.5. Especially wide open the difference is huge.
The late "silver" FA's are impressive lenses. Plasticky, badly built (glued optical cells), somewhat wobbly, but optically outstanding, especially considering the price.
The 28-105mm shines more than the others. It's better than the previous "dark grey" FA (f/4-5.6), and I guess also better than the Pentax/Tamron with identical specs and internal focusing. Always used it with the original hood, and found that it shows a very good control of inter-reflections, high contrast, and incredible, punchy colours. In certain pictures the "pop" reminds me of the 77mm Limited.
I have seen a quick comparative test on this forum, where the old FA was pitted against the new D FA. Not a huge difference, I remember. If optical performance is the only consideration, I guess the late FA is the only similar Pentax zoom that plays in the same league. The new D FA has other qualities though, don't get me wrong. I just can't justify the expense for the new one considering what I'm getting from the old
Another Pentax lens more or less in the range is the 28-70mm f/4 AL. I have used it more than others, and I have been quite happy with its performance. The only problem is that I never used it on the K-1, not a single time, so my experience is limited to the crop format. If f/2.8 is not required, and a small form factor is welcomed, it could be the best option, price/performance wise. It's easy to find, cheap, and it's usually in great conditions. Not all the zooms of the same vintage are as easy to find in good conditions. Another very good zoom, the Pentax-F 70-210mm, has often problems ranging from fungus to separation of cemented groups.
I'd be happy to try the Pentax-F 24-50mm. It should be better than the FA 24-90mm cause it's way less extreme. I've read positive reviews on this site, and should be a lot cheaper. It should be considered, a pity is not a very common lens.