The two major "reactors" in use would have to be Calcium and Fluidized.
The calcium is for obvious reasons, but if your tank does not have a high calcium demand then there is no need for you to spend the money on one.
The fluidized is for holding whatever type of filtration media you wish to put in it.
Two of the more popular ones are carbon (obviously) and iron based phosphate removers such as ROWAphos and Phosban.
You do not NEED this type either unless you are having PO4 problems then they can be a great aid in lowering your phosphates while you are figuring out where they are coming from and how to fix them.
Then, many including myself, still run them to lower PO4 as much as possible.
hth,
Sean