Tough call as there are many kinds of wildlife. I suspect there is no perfect wildlife lens. What might be "best" for my needs might not be "best" for yours. In any case I assume you mean the best Pentax lenses for wildlife and I will limit my response to this brand.
My longest lens is the M 400/5.6. This is an older lens and lacks ED glass and hence is prone to CA and PF. As a result it won't make anyone's best list unless it is a best you can afford list.
For my money wildlife lenses begin at 300mm and this is entry level. This is especially true for birding. You never seem to have enough length or lens speed in this category. The best in this class, of course, cost a lot of money as a quick glance at the lens review area will confirm. It will set you back several thousand dollars for a 600mm lens. The best wildlife lenses will be found in this 300~600mm focal range and are out of reach for all but the wealthy amateur and professional photographer.
In the Pentax line any of the following might be included in a Pentax "best" list due to ED glass, (IF) internal focusing etc. We should probably divide the list into auto focus and manual focus. None of the lenses in either group will be cheap but the best never is.
Manual focus "best" list
M*300/4
A* 300/4
A* 300/2.8
A* 400/2.8 - I really want this one.
A* 600/5.6
A* 1200/8 - a seldom seen speciality lens without a review in the Forum at this time. It is so rare as to almost be almost irrelevant.
Auto focus "best list"
All the FA primes 300mm and above get good reviews
FA* 300/2.8 appears to be outstanding.
FA* 400/5.6
FA* 600/4 – I want this one as well.
DA* 300/4 appears to be an excellent lens and many await the arrival of a DA* 400.
Forum members such as Pal Jensen, Ben Edict, Wheatfield and a few others can probably give a better opinion on the “best” Pentax lenses in this category as they own and have used them. I assume these Pentax telephotos perform as well as their Canon/Nikon counterparts. Pentax lenses with shorter focal lengths certainly do. For most of us, alas, this sort of glass is pretty exotic stuff much like a Ferrari. We might see one from time to time but it is unlikely we will ever get to drive one.
Cheers
Tom G
I thought I might add a best "affordable" wildlife lens list. By "affordable" I mean a lens around $400 or less. This is an arbitrary number but I had to pick one. I am limiting this to Pentax lenses some of which I own. All are manual focus and none have low dispersion ED glass nor do they have internal focusing (IF).
Best "affordable" 300mm
SMC Pentax K 300/4 - it has PF/CA issues but is a great buy. Pretty fast at f4 and it lacks a tripod collar but can deliver very good images. The lens has it flaws and detractors but represents the best "value" in the 300mm category.
SMC Pentax M 400/5.6 - again, PF/CA in high contrast situations but the extra length is great for birding.
The A 400/5.6 is a better lens than the M 400/5.6 and will focus more closely. I have seen it listed anywhere from $1,500 to as low as $700 so it doesn't fit my "affordable" criteria.
SMC Pentax K 500/4.5 - will certainly cost more than $400 but is worth considering (I have) as a bargain wildlife super telephoto. It shares PF/CA traits mentioned above as all telephotos of this vintage do. It will, however, get you up close and personal from a distance.
Someone with more knowledge of Third-Party lenses: Tamorn, Sigma, Tokina etc. might want to comment on those brands for wildlife.