There are a couple of different options depending on how you're stocking the tank and how much you're willing to invest in money, maintenance, and setup. Canister filters are my preference, I have a Unimax 150 on my 29 gallon and I love it. The filter media is much more customizable, and you can fit so much more in it than a normal hang-on-back (HOB) power filter will take. Some canisters can hold heaters, and the Unimax 150 even has a UV sterilizer bulb in it. The best part is, the filter is completely silent, and there's no waterfall noise because of the design of the spray bar, so I can have the fish tank in the same room as my television. HOB power filters will be noisy, and don't have as much room for filter media, or much customizability (you have to get specific replacement filter cartridges), but they are cheaper and easier to clean. Once you decide on what type of system you want, and how much you want to pay, then you can narrow it down to a few units. Look up reviews to determine which one will suit your setup best.