The favourite original set: Holiday Hare '98 Evening
The favourite custom sets:

• Swamps of the Sleeping Jaguar Evening (Evening tilesets are fun )
• 7th Lava Fall

In a tileset I value it's appearance quite much. The levels must look nice, and heck, with Swamps and the HH'98 you can create unearthly beautiful levels. Of course the playability must not suffer. Some tilesets, such as Egypt, could be a lot better with more carefully made masks. Also, I like tilesets that look hand drawn (here's an explanation to my Agama obsession ). Exception to this is the 7th Lava Fall set, which is spectacular when used properly.