I don't really want to say the ones I'm working on
, but of the sets I don't actively collect, 1916 and 1917 black n whites are my favorite. Like Jeff said, great looking cards, all the stars and lot of fun ad backs.
Also
t207s
Diamond Stars
Exhibits (getting more love lately but you never see any posts from someone trying to complete say 1928 Exhibits)
Kind of seems like sets that are relatively easy to find and have just one back don't get talked about a lot, but maybe there just isn't much to say about 1934 Goudey or Diamond Stars or e93s?