Rena XP3, if you can afford it. It's really quiet and I think you will be happy with it. I like my XP1. Plus, it comes with both a nozzle and a spray bar so you can choose which one you like.
If you don't want to spend that much, I'd go for the Aquaclear, because it is more quiet, more customizable, and moves more water than the Emperor. Don't get me wrong though, the Emperor is also a great product. I've never kept up much of a maintenance schedule on mine, yet they are EXTREMELY reliable. However, I don't really like the "new" Emperor. I think the mid-level intake strainer is stupid, and I hate that they took out the little windows in the cover that let you see if your Biowheels are spinning. Plus, they raised the price another 10 bucks or so (at least, the wholesale cost that LFS's pay)!!!