Popular for whom? And on what basis? Should I venture a guess, I imagine Reverso is, by far, the model most readily associated with JLC amongst watch buyers in general. It has the longest and most enduring history; while other models, e.g. the Memovox or the Duometré, might be said to be more original, they remain too niche or too expensive for wider consumption. Vice versa, amongst the purist community it would probably be between the Geophysic 1958, the Polaris and the Deep Sea Alarm (I am particularly partial to the version in your picture). Without having looked at the statistics of old posts on this forum, my bet would probably be the 1958 Geophysic; although the buzz has calmed a little, the coverage and discussion from re-release in 2014 and onwards was huge. Then again, this may all be subjective, since it happens to be the three models I am really lusting for.