I have Fritz 12 and have played numerous engine tournaments and the free Rybka 2.3.2 beats Fritz 12... I believe Rybka 4 is the best UCI engine out there followed by Rybka 3. As far as GUIs go, Fritz is better IMO than Aquarium 2010.
The best chess program.
I don't get what the other program in fritz is.
The Fritz playing program consists of the Fritz interface (GUI) and Fritz engine. It usually comes packaged with other engines, such as Crafty and Comet. Other playing programs sold by ChessBase include the Fritz interface. For example, Hiarcs 12 contains the engine Hiarcs and the Fritz 11 GUI. The Fritz interface also supports UCI engines (Universal Chess Interface), making it possible to install hundreds of engines within the program.
Chessmaster is similar: the GUI is called Chessmaster; the engine is The King. Chessmaster fails to support other engines. The King is the only one that works within its GUI.
Folks that run engine tournaments to determine the relative strength of different engines must use a modified UCI compatible version of The King so the engine can run in Fritz, Arena, or any of several other GUIs that facilitate engine-engine matches.
Thanks
This is one of the best posts I've ever read on Chess.com.
I agree.

Whats the king?
The Chessmaster chess engine is called The King, written by Johan de Koning of the Netherlands. It was introduced in Chessmaster 4000; earlier releases featured a chess engine written by David Kittinger.
Pasted from Wikipedia's "Chessmaster" entry. I do not always trust Wiki, but in this case, the information is generally known and accurate. References to Johan de Konig are made in Chessmaster's program manual and the files accessible from within the program.
This is one of the best posts I've ever read on Chess.com.