As an aquarist I have tons of material laying around (large cell, medium cell, small cell, poly fill, foam, you name it) and my favorite so far is the Marineland Rite-Size bonded filter pad. It's actually two layers, blue and white, with the blue being a bit thicker (I use the blue). When folded in thirds and stuffed into the cartridge I get every last bit of the
juice I drip onto it. It's about $5.00 for 312 sq. in. My second favorite is the generic grey small cell air filter foam. It absorbs and wicks really well but it's not so forgiving when it comes to size. Too small and it can leak, too big and it compresses too much to wick. When it's cut just right, however, perfection!