I included the CK too because in the original version 15 sample field were included under the name "Boulder Dash IV".
Give your vote above... and euhh, feel free to comment on your choice!
Of course I'll start. I have played BD1 almost infinitely often. I remember when I played it for the first time I always got stuck at cave C (!), and later at J, L and N. BTW - I was 8 y.o. at that time... but very happy when I managed to complete the whole game (read: level 1) for the first time!
After playing BD1 I got the famous package "Boulder Dash 16" by No One with 16 BD games included (but the caves were modified wrt the originals). So I played BD3, 4, ... and later on BD2. Although I was certainly impressed by all these games, it never was the "magic" of the first BD game.