For the classic Helios “swirly” rendering, the community answers point to the earlier 44 and especially 44-2/44-3 versions as the ones most associated with that look. The later 44M-4, 44M-5, 44M-6, and 44M-7 are generally described as more corrected and often less swirly.

So if your priority is the signature bokeh rather than maximum sharpness, a Helios 44-2 is the safest pick, with 44-3 also commonly grouped with the swirly versions.

One important warning for your Nikon D600: adapting M42 lenses to Nikon F is problematic if you want infinity focus. A simple adapter usually won’t reach infinity, and an adapter with corrective glass can reduce image quality. As one answer notes, on Nikon F these lenses are often practical mainly for close-up or macro use unless you accept those compromises.

In short: choose a 44-2 (or possibly 44-3) for the strongest classic effect, but be aware that Nikon F adaptation is the bigger issue than the version differences.