[Quote]However I was looking for something more practical, with real tests made on the k1[/quote]
Are you aware of the lens tests on the US Pentax forum? They have a dedicated space for user reviews/comments on most of the old Pentax lenses whereas that facility isn't available here.
e.g. https://www.pentaxforums.com/userreviews/tamron-28-75mm-f2-8-af-xr-di-ld-macro-s...

You have to remember that the K-1 is still a premium product where the buyers will mainly go for the new dedicated DFA lenses so suspect not many will have those older lenses to test and then come here to write a review on them as there isn't a dedicated space to write it.

By all means everyone, I'll happily be proved wrong!

None of those, sorry!
Most, or should that be all, of the old legacy film lenses will work with the K-1. Obviously results will vary so you need to define 'best'. One lens may produce an effect the photographer likes whereas someone else thinks otherwise. An APS-C example of this is the Sigma 10-20 where some love its natural vignetting, others do not.
Then are you looking for ultra-wide, wide, normal, telephoto, long telephoto? Prime or zoom?
For starters though I'll suggest the three FA limiteds, 31mm, 43mm, 77mm (or even the DA70mm)
Any of the FA or DFA macro lenses; 50mm, 100mm are useful to have, not just for macro.
A fast 50mm prime, the FF standard lens.
Then do you want to experiment with the old manual focus lenses of the 1970's and 1980's?
They'll all work and provide fun for some to experiment with, others will just get the new DFAs and be done with it.
So you see, to answer your question as posed is a bit difficult! On top of that there is no guarantee you'll easily find a copy of great old lenses as you can't always order a new one.
Another thought to add is that some APS-C lenses are more than usable in full frame mode if you are prepared to tolerate some edge cut-off or vignetting. Several such as the DA35 and DA50 work superbly well in FF mode; two bargains for sure.
The main issue with older glass seems to be that chromatic aberration is more likely to be noticed as the lenses are designed to focus onto film rather than a sensor and pixel peeping is more likely to take place than on a 35mm print or slide so issues get noticed when they were insignificant before. Having said that, the effect of this is less marked with full frame compared to APS-C as in effect the smaller sensor is causing the image to be magnified/cropped in camera. Whether this is an issue or not depends on the user.
As to legacy zoom lenses, some of the old Pentax zooms remain really good to use but some of the cheaper consumer 'kit lens' zooms are not. Do some research and you'll be OK.

I'll go along with that!

Ditto.

Nigel, I keep on agreeing with you!

Mike, sorry if I misunderstood what you were trying to get across and you're right to answer the question. However when choosing a camera, just looking at one aspect of it, particularly a point that is generally recognised to be a weak one for the brand, doesn't mean it is that bad to not be usable for the purpose. Some may think it is, some not.
A camera could have the fastest AF of any camera ever invented but be totally useless in all other respects such as when all its functions have to be set from a complex menu system on a screen that can't be seen when its sunny! By time the autofocus has been set the subject would be long gone.

I'm pretty sure they must be working on it as I can only presume that's the idea behind the KAF4 mount and the new PLM lenses.
However there are some of us who are quite content to use lenses produced before autofocus was invented. At least Pentax make that relatively straightforward so don't see the problem.

Mike, there's more to a camera than its ability to track and focus, especially when one doesn't use that facility! What's right for you may not be right for someone else so there's no need to be sarcastic like that.
You buy what's right for you and I'll buy what's right for me.

Edit: If that facility was offered for a premium in a camera I wouldn't go for it as I have no use for it. Similarly with enhanced video features some call for. I'm quite happy with the offerings as presented. I dare say though that both will continue to get enhanced, hopefully without impinging on the other features and requiring a totally different camera with a different mount as it would then only be a Pentax in name.

Yes, a full frame comparable to the KP would do well, even it it had to be scaled up slightly to make room for the larger sensor. Still happy with my K-1 though so not hankering after such a thing.

Your best bet is to go and try some somewhere, See how they feel compared to your EOS. Take a memory card with you and compare the results. See how you find the focussing by comparison. Only then will you know if its right for you.
I found the basic 18-55 lens supplied with the Pentax streets ahead of the equivalent Canon, and that lens isn't particularly well regarded. I photographed a bar code which on the Canon came out just a blur but was perfectly clear on the Pentax (both taken with flash to avoid camera shake.) Then the Canon consumer telezoom which I also had (around a 75-300) was so poor at 300mm it was almost unusable. I didn't think much of the brand that so many were enthusing about. So EOS sold and back to Pentax from then on and all my old film lenses gained a new lease of life!
Also don't get me started on a film EOS300, it ate batteries for breakfast compared to the batteries that had been in my Pentax film bodies for a decade, and still worked!
No way would I go that route again.

I agree Nigel.