xenia(its a weed), mushrooms, zoa, certain sps grow quite fast too! but at the end of the day its all up to your water parameters and the stabillity of your tank!
Kenya trees, GSP, and digitata also grow fairly fast, but I agree it is mostly up to your lighting, water quality, and flow rate. I have kenya tree frags that dropped two weeks ago that are now larger than the frags I gave to the LFS 3 months ago.
my fastest is my red carnation has two branches that have grown an inch in a month and is growing a bunch of smaller braches. its needs a ton of flow though