http://www.jbjlighting.com/prod_28g_nano.html
This link is for the 28 gallon JBJ Nano Cube. I am not sure what exactly the best is but I have owned the 12 and 24 gallon nano cubes before.
The overflow on the nano cubes (Not sure about the 28 I think they fixed/modded it) doesn't have a surface skimmer. This allows oils to gather and float on the top, reducing light and just adding a layer of slime. There is a little 10$ insert that literally looks like a old cassette tape case that has been cut to perform the role of a surface skimmer.
The return pump seemed fine but I upgraded it for better flow and it seemed as though that was a common mod.
I also bought a Hydor for the output. The Hydor spins and changes up the current. (Really cool device, check it out no matter what Nano you end up at.)
The last really big weakness is that the Nano tanks have a single cord to plug in, for the Night Light and also the 2 sets of lights in the hood. So you can't really use a timer. I replaced the ballast with one that had 2 cords instead of one and also a single cord for the moon lights so that they could all be operated by hand.
I ended up spending way to much money on this tank with small mods. After replacing the pump, adding the surface skimmer, adding a hydor and replacing the ballast I really liked it. (It still had lighting for low light corals)
But those mods cost money and it seemed as though, after one was added another would be waiting. They are expensive too. Go to nanocustoms.com and look at the hoods they sale. Can almost keep your 55 for the same price.
(I have gone from a 55 in college to a 35 hex when I moved home. After that I moved into a 12 gallon jbj nanocube. (Upgraded the skimmer, output and pump) Then a 24 gallon jbj nanocube (Upgraded the Skimmer, output, ballast and pump) Now I have a 50 gallon with 30 gallon sump.
I had to have more room. JBJ SUCKS without the skimmer (overflow) mod. The ballast is frustrating, who wants a single cord to control 3 different lights? Am I the only one that uses timmers? The Pump is a mod that is easy and everyone seems to recommend and do. The Hydor output is awesome. No matter what Nano you get this should be looked at.