For the money? That's tough...most people LOVE the ATI sunpower and will say that ranks 2nd to the ATI Powermodule. However, I know alot of people who are very happy with the Aquactinics that are slightly cheaper as well. The new TEK Elite is the one all the t5 guys are waiting for. They are cheaper than both ATI and Aquactinics and seems to correct their biggest criticism on their previous fixture, fans.
But IMHO, if you have some electrical knowhow and handy, buying a used/new 660 ballast and putting your setup up together is probably the best performance for the money.