Canisters are my choice for tanks forty gallons or larger, especially for show tanks, as they can be completely concealed and not detract from the appearance of the aquarium. Because canister filters force the water through the filter media, they are particularly good at removing particulate matter. Some canister filters even have diatomic sleeves to allow for special filtering situations.
1. Eheim Canister Filters
Boasting a high throughput rate and durability, the Eheim is the quietest canister on the market. I like the compact design and easy to handle latches. My only complaint is the lack of self-priming, a common problem in canister filers. However, priming is vastly improved in the Ecco and Pro II series. Although pricier than other brands, I've found that over time Eheim out performs them. 
2. Magnum
Magnum balances price with some of the best features in a canister. It is self-priming, quiet, and offers convertible filtration modes, including the option of adding a matching Biowheel system. The clear canister makes it easy to see the condition of the filter. Optional gravel cleaner is a nice perk to an all around great package.
3. Fluval
Fluval produces one of the most economic, yet efficient, multi-media filters available. Learning a lesson from their older models that were too tall and unwieldy to service, the redesigned models are compact, and sport larger latches similar to the Eheim products. Priming is still a headache, and they tend to be noisy if not properly filled. In spite of that, you can't go wrong with a Fluval.4. Proquatics
If it weren't for the lack of performance data, this filter might rival Magnum and Fluval. The unit is built with the same top-notch features, such as self-priming, easily removed media basket, large easy to release latches, and top mounted swiveling flow controls. So far I've been very impressed with its performance.5. Rena FilStar Canister Filter
Although less well known that other canisters, the FilStar has many of the same features. The self locking water shutoff system is a nice touch, and the price is very competitive with other canister filters of the same size.