Depends on what you want to spend to get the "best" and what you plan to keep in the tank. I have a Tunze 9002 in my 30L tank and love it. It did great when I had SPS in the tank. It is an in the tank skimmer but does a great job and is quiet.
If you want top of the line look at the Warner Marine H1 or H2. There is also the Octopus HB 800S. Deltec also makes a good one, the MCE600.
