I need to chime in here from the UK and say that although the 600 was great looking - the 2.04 Rom's caused no end of problems in getting games to work so in that sense (along with the 500+) it was the most unpopular. These days though (as I think Camy said) you can have an A600 with 3.1 Roms and Compact Flash solution that works as sweet as a nut and I'm now seeing these as being really desirable (my friend has one like this and it's tops!). So you can't play AGA games on it but no REAL loss there
For rarity (which obv is not the same as unpopular) I would say that the A4000 was the rarest of machines where I lived at the time - probably due to their price. The A1200 was probably the most popular of all the machines as everyone I knew upgraded their 500's for an A1200 and I think that still, these days the 1200 with the compact flash solution is a fantastic machine (especially if you add an indivision to it!).
John
