Certainly in Britain the 1936 (as usual, following the Abdication) is the most common. 1928 and 1929 are plentiful too, being first two years of essentially the first new reverse since 1902. 1935 I would say next, though not very different from 1931 and 1933 also. 1932 is the third hardest, though quite a bit easier than 1934, and 1930 the hardest. (I'm talking high grades here - none are rare in low grades not even 1930.)