For me it's a
very close call between Fire and Crystal.
Fire is the most usable tileset IMHO. It has the biggest number of pieces and lots of different shapes. Aesthetically, it also has (arguably) the most recognisable theming of all the original tilesets. It does a great job of looking like it's actually hot! Crystal, on the other hand, is its spiritual opposite - cool, mysterious and almost space-themed. It's heaven, whilst Fire is hell. Or, if you prefer, the two could be seen as Fire and Ice
I chose Fire because I think it deserves a few more votes, but I'd happily have voted for Crystal as well since it's arguably the best-looking of the sets. It also formed the basis for much of the willlem_lemminas tileset.
Suggestion: add the
Oh No! tilesets to this poll as well, and maybe allow users to vote for more than 1 set. Say, up to a maximum of 3.
Good topic!
EDIT: Incidentally, my top 3 favourite levels from L1 are in Fire and Crystal:
Mary Poppins' Land,
Take A Running Jump and
POOR WEE CREATURES!