The electron phonon coupling mass enhancement factor $\lambda$ is a measure of the strength of this coupling. This quantity can be measured experimentally. For instance, Pb has a factor of 1.55 whereas Nb is ~1.0 - 1.2 [source]. Because of this, Pb is typically considered a material with strong electron-phonon coupling.
What is the material with the strongest $\lambda$ known so far? Is Pb the highest ever measured?
Edit: I found this paper which compiled experimentally measured $\lambda$ at the time (1972) where we see some Pb alloys having values up to 3.30. I wonder if there is a general table for all measured values of $\lambda$.
