That depends on how many fish you have, and how much you want to spend.
The aquaclear ac110 is a nice HOB (hang on back) filter, it will let you use higher quality biomedia.
A canister filter like the Fluval 305 would also allow higher quality bio and mechanical filtration, as well as run quite.
You could also build an above the tank sump or traditional sump.
However this is all Dependant on the fish you have and the bio load.