I guess it depends on what you want to do with it, but I like the Hagen Elite for small tanks. It has a flow control, venturi (good for DIY CO2) and is tiny, quiet, and cheap. Petsmart and Petco sell them too, although I don't see them on their websites.
I've tried loads of different types and makes, the smallest and most of all silent is the Koralia Nano. You won't find anything much smaller. The only problem is that it's quite strong, possibly too strong for a smaller tank. But you could always point it upwards if that's the case.
I picked up a tiny submersible filter / powerhead thing on amazon for like 10 bucks and used in a 2 gallon tank for a while had to turn the flow all the way down to keep from blowing the fish into the glass! lol
Just to provide an update on the smallest powerhead out there
Rio Plus 50 Aqua Pump - 69 GPH $15.10 on amzon
Product Dimensions: 2.2 x 1.8 x 1.4 inches ; 9.6 ounces
The new and improved Rio Plus 50 Aqua Pumps and Powerheads are versatile water pump systems designed with high efficiency and reliability at a lower cost. Designed with next generation magnetic-rotor technology and a new impeller with ceramic bearing and shaft, the Rio Plus Aqua Pumps and Powerheads offer the best high-performance solution in the aquarium industry. Versatile: water circulation, fountains, protein skimmers. One of the most energy efficient water pumps that combines low energy consumption with silent cool operation. Dimensions: 2.4 x 1.4 x 1.8 inches. 5.3 watts. Maximum head: 2 feet.