If your looking for a Hang on back filter, then go for the Aquaclear 500, it can really hold a lot of media and turns over a very good volume of water (rated at about 500gph, but thats optimal at best). It's easy to clean out too, if you need to, just pull out the basket, change the carbon sock, and sponge and put it back in. Takes all of 3 minutes to do.
You can also adjust the flow rate on the AC 500, though on my 20 gallon high tank I just leave it on high, and it works great for me.