Agama, Disguise, Blade, BlurredD, Nick, Dean, Mezz, Dethman, ET, Ice M A N, Craccoboy, Haze and Skulg are some of the more popular tileset creators, but look at it this way - tilesets get used because they're good in some way. For example...

1. Really good graphics.
2. Just the tile or tiles needed for this level.
3. Lots of possibilities to create a really cool level.
4. Good, but underused enough to make a level with it.

Or, some people will use a tileset simply because they made it!