At first I voted only dirt and thought that fire, marble and crystal are equal and pillar pleases me least of these. Now I voted dirt and also crystal, which is beautiful, mysterious and a little gloomy tileset, but I've started to like it more and more now, probably due to some nice crystal style custom levels I've played.
Dirt is of course very memorable and nostalgic style because the starting levels of fun, tricky and mayhem are dirt styled and the last level of the whole game is a dirt level. I find this bumpy underground tileset laid-back and sometimes even pretty: for example I've always liked the idea and the layout of taxing level "Upsidedown World".
In addition, both crystal and dirt are the least used tilesets in original Lemmings game.