radio astronomy - Is there a single, most prominent helium line?

I am unaware of there being a single Helium line that is as prominent or useful as the 21cm line is for Hydrogen.

The 21 cm line is due to a hyperfine transition in the 1s ground state of hydrogen. It is notable due to its strength, and also its usefulness for mapping neutral hydrogen in the universe, which is impossible in the optical since it is absorbed by neutral Hydrogen. It was in fact the first such spectral line observed in the radio part of the spectrum, and is so fundamental that the Pioneer and Voyager probes used the length of the transition as a time and distance unit on the plaques they carry, so that potential extraterrestrials could decode the messages.

There are many more spectral lines in the radio from heavier elements, and a whole field dedicated to observing and understanding them: radio frequency spectroscopy.

For Helium in particular, hyperfine lines can also be observed; the hyperfine transition of the $^3$He isotope has a wavelength ~4mm. The same isotope also emits a recombination line, where an electron recombines with an ion and falls to lower energy levels, emitting a photon in the process. The wavelengths of such recombination lines for Hydrogen like atoms (singly ionised in the case of Helium, so that it has a single electron) can be estimated using the Rydberg Formula:

$$nu = R_{m}cleft[frac{1}{n^2}-frac{1}{(n+Delta n)^2}right]$$

where $R_m$ is the reduced mass Rydberg constant, $n$ is the final state and $Delta n$ is the change in state.

Estimating the strengths of such lines is a little more complicated, but there are details here.