The posted tilesets are good, but they are pretty large to someone used to the default tileset and don't scale smaller well. I would recommend Alloy. I initially ran into problems getting the transparency to work, but opening an re-saving the image seems to fix the issue. I've uploaded my fixed version of the file.
It seems to be basically identical to the default tileset, but with stuff edited to be 12x12 instead of the default 8x12 or the packaged square's 16x16.
To install it, just download the above image, from there or from the wiki, save it as
...\data\art\Alloy_curses_12x12.png, and replace the lines in
...\data\init\init.txt that say
[FULLFONT:curses_640x300.png], lines 30 and 39 as of version 0.34.11, with