I would have to say biggest bang for your buck would be the aquaclear 500 sold at
http://www.bigalsonline.com its only $30 and you really can't beat that because they are reliable( I have two going on 3 years that have not had a single problem) and they are very quite. They also push a lot of water and are good for messy cichlid tanks.
As for the best filter you can buy, that depends on what you are looking for. If you looking to really clean the water I would say a micron canister filter, its really good to mix them to get the benifit of all of them.
Good Luck
Renzema