Chemex Filters FS 100
The whole thing with these filters is that they are thicker than most - this means that a lot of the coffee oils are held back by the filter, so you get a super-clean cup of coffee as a result.
The square filters come in packs of 100, and are suitable for all Chemex bigger than the 3 cup . These ones are pre-folded, so you don't even have to think about it. Just pop it in and brew.