it generally depends on what kind of "best" we're looking for.
for example blood knights, while stat wise the most powerful in terms of attack are stupidly expensive and have significant drawbacks, they can be lured thanks to frenzy and they crumble like no tomorrow with their low leadership value. they are also VERY expensive points wise, only marked chosen are in the same points range.
Out of the three choices however... I’m quite torn. the knights of the blood keep are by far the most dangerous in terms of pure damage, but are slower than the grail knights and lack the "dead'ard" nature of chosen.
Grail knights after all sport magical attacks as well as being immune to psyc, all being champions and having the lance formation, gives them some significant advantages.
Knights of chaos have lots of versatility with the choice of the 4 marks as well as having the most survivability and staying power of any of the knights, they don't crumble or loose any hitting power in following turns. each mark has its downsides of course (mostly price in points though luring make khorne suffer) but still
I reckon I’ll have to abstain