@Omega_Variant that’s a tough one; it cannot be Chaos for Chaos always collapses in upon itself. it cannot be the Imperium for it is routinely and most affected by the curse of Chaos.
da Orks are like unto Chaos in their inability for long-term widespread cooperation, thus cannot end in cohesive Ork victory.
among those presented, i’d say only the Necrons have a real shot. they’re the oldest still-extant race by far, far older than even the Eldar which makes them positively ancient compared to every other race, and possess the only means of FTL travel that eschews the Warp as a medium.
the Tyranids might have shot were it not for the Necrons for i do not believe that the Tyranids, being an all-consuming hive form of life, could eat the curious alloy of which the Necrons are made and the lynchpin of the Tyranid strategy is the repurposing of dead biomass.
so as far as an ultimate-victory, short of the Emperor coming back, i think for no other faction is this even possible. however, for various reasons i think it will never come to pass i am not a Necron player i like Chaos, Adeptus Mechanicus and Adeptus Sororitas. thank you friend.