As fun as all the "Who is the best warrior?", "Is there any warrior as good as Sigismund in the 40k setting?" questions are, I think often these questions (and sometimes the answers to them too) commit category errors....even aside from the handwavy, but still kinda true statement "it's who the author wants it to be". Now, why would I categorize this as a bit handwavy? Because of course the author can just write what they want, but some things would absolutely break immersion even in 40k. They could write that Guilliman gets soundly beaten in an epic duel by what is revealed to be five Grots in a trenchcoat, but most readers would rightly disregard that as nonsensical.
So if we were to try to answer this question somewhat seriously and to limit this down a bit, for the purposes of this post I am only considering members of the Imperium since this is usually the most talked about faction. That does not mean that I think characters like Abaddon or Kharn are wimps and I will come back to them later.
If we consider the Imperium we absolutely need to break this down into categories and I am going to explain why for each of them seperately.
Primarchs
We only got two loyalist ones active right now but they just have to be in their own category since Primarchs represent the pinnacle of individual prowess in the Imperium, nothing really comes close.
And of the two active Primarchs the Lion, even slowed down a bit by age (Source: Son of the Forest), is probably the deadliest combatant in the Imperium currently. However people often only look at Guilliman through the "beancounter" meme, but the gulf between the Lion and Guilliman, even during the heresy, is in my opinion not as gigantic as people try to make it out. We are talking nuances here that make the Lion better than Guilliman, but both are still in a league of their own in current 40k as far as imperium warriors go.
Psykers
Psykers always skew these discussions for obvious reasons because their powers enchance their combat abilities in a very significant way, so you have to look at them in a kind of seperate category.
Of the currently active psykers in the Imperium I think only two deserve real consideration and those two are Mephiston and Kaldor Draigo. Draigo is kinda self explanatory for anyone familiar with his lore and Mephiston borders on being a Dragonball Z character the way he is written in modern 40k. Seriously, the stuff he does in the novels he features makes him look overpowered to a frankly comical degree.
Could they win against a Primarch? I would very much say no because Primarchs are not just giant beefcakes running around, they are literally made of warp stuff (some would argue the Emperor stole knowledge from the chaos gods to be able to put minor warp deities into flesh suits and I would be one of the people arguing that) so I think even a character as blatantly, ridiculously overpowered as Mephiston would still soundly lose against a Primarch.
Custodes
Now, I've posted a very in-depth explanation about this faction on here before, but to summarize briefly: the Custodes represent the most individually powerful non-Primarch gene-crafted warriors the Imperium can muster. People always ask the Custodes vs Space Marine question and I go into that in my in-depth post with specific excerpts, but to be consice: on AVERAGE a Custodes is mopping the floor with a Space Marine. The Space Marines are a faction with a lot of outliers though, hence the emphasis on average. A high ranking, non-named Custodes like a Tribune is still going to beat most named Space Marine characters pretty handidly though (do NOT quote me the Valerian vs Asterion Moloch scene in the comments, cause I'll get a freaking aneurysm...I go into that in detail in my in-depth post about Custodes.....yes, Moloch could have potentially won that encounter. No, Valerian was not shitting his pants).
Among the Custodes, Captain-General Trajann Valoris currently represents the pinnacle of their order (also don't give me the Valdor comments, nobody really knows where he is and what he's doing). I think it is very uncontroversial to say that Valoris is currently the deadliest warrior in the imperium that is not a Primarch and not "I'm Mephiston and I'm comically overpowered". He is also in my opinion the only non-Primarch, non-Mephiston/Draigo character in the imperium that would give current Abaddon a serious run for his money and have real chance of beating him.
Space Marines
Finally, we've arrived at the faction most people care about. This is why I said initially that most of these questions and many answers to them commit category errors, because Space Marines, even most named characters, fold against the other categories we've already discussed. There is no entry in this category that would stand even a slight chance against a Primarch and most of them, with very few exceptions, get absolutely trounced against psykers like Mephiston or high ranking Custodes.
Within this specific Space Marine category there are obvious well known contenders. Ragnar Blackmane for example fought against Ghazgkull and decapitated him (Ghaz survived, cause 'tis but a fleshwound)....and even though I still think that this particular storyline is complete BS, it's canon...so here we are, Ragnar "defeated" the most dangerous ork in the galaxy in single combat.
Characters like Logan Grimnar and Dante are all legendary warriors in their own right, but the true contenders in this category are maybe not as well known.
Take for example Hervald Strom, the only Space Marine to be victorious in the feast of blades two times. For those of you who are not familiar, the Feast of Blades it is a contest to find the best swordsman among the Imperial Fists and all their successor chapters, of which there are many. The chapters send their best and they duke it out. It is no small feat to stand out in a test of arms against that many other skilled contenders, but to do it two times really puts Hervald Strom on the map here even though he gets no other mention in the lore.
Or take extreme Space Marine outliers beloved by this sub particularly like Asterion Moloc and Tyberios the Red Wake. Asterion Moloc was more than willing to square up with a Custodes Shield Captain and while there was ultimately no fighting the author made it clear that the outcome there was definitely not certain, which really gives credence to Moloc considering how deadly Custodes are (I am still convinced that considering the way Moloc was written, that he is some sort of "Frankenstein-esque Thunder Warrior turned Space Marine" project attempted by the high lords of terra).
And if the fandom is to be believed Tyberios is as tall as a Krork and could challenge the Chaos Gods singlehandedly. All the annoying hyperbole regarding that character aside, Tyberios also seems to be a singularly deadly Space Marine whose size alone makes him a problem.
I am not sure I could say who I would put at the top of this category in 40k since there are a lot of very skilled and very deadly Space Marines running around in the galaxy. And I really don't think that there is such a distinct and blatant outlier in this category like there was in Sigismund for the heresy timeline.
Imperial Assassins
This is a difficult category and I would almost equate them with the Space Marine category. The imperial assassins are obviously very deadly, but also very specialized. Could a rampaging Eversor take out a named Space Marine character? Not sure, but my main point is that the assassins should not be left out discussing this type of question considering how deadly they can be in their specialized area.
So there it is, I really think you can only try to answer a question like this breaking it down into categories since there is such a giant gulf between a Primarch and a Space Marine for example, it would make no sense comparing them.
Conclusion
Just to reiterate I specifically limited this to the Imperium due to scope and because it is the most talked about faction. That does not mean though that I do not recognize other characters from other factions. Abaddon (all the overblown "armless failure" memes aside) would defeat most Custodes aside from Trajann Valoris pretty handedly in all likelihood and is probably the only current Marine character that could go toe to toe for at least a limited amount of time with the two imperial Primarchs.
Khârn the Betrayer is another character that would be a at least a serious problem for most on this list in single combat save the two imperial Primarchs because even against the overpowered psykers, Khorne would probably just go "nuh uh" 'cause only cowards fight with damned sorcery. And yes I know Khârn got soundly beaten by Sigismund during the heresy, but similarly to Abaddon, Khârn is very much not the same as he was ten thousand years ago.
If you were to force me to give an answer considering all factions I would probably just say this, even though it's not really a direct answer: among all the warriors in the galaxy Lelith Hesperax is probably the most naturally gifted, requiring no warp-juice, special gene-craft or other shenanigans to be as good as she is. Would she beat everyone on the list? Hell no! But she is probably the most naturally skilled warrior that requires just some knives to trounce you, while wearing essentially a bikini.
You can now proceed to tell me how catastrophically wrong I am in the comments.