Aquarium GUI was made specialy for Rybka 3, but can run others engines.
I will buy Aquarium.
The best book for Rybka 3, will be she´s book, Rybka 3.ctg, from Jeroen.

Paulo Soares

Yes, Aquarium seems to be the first choice.

Hiarcs has no own GUI, so hiarcs 10 and fritz 10 have the same GUI I would think. Fritz 11 GUI and Fritz 10 GUI seem to be similar. Their advantage is that they support online engine-engine play, which Aquarium does not (at least in the first version) and Rybka's ctg book can not be modified in Aquarium GUI (but it can be opened in Aquarium!). If you want to play on chessbase's server engine vs. engine matches, also the Fritz 8 or Fritz 9 GUI is ok (and cheaper than Fritz 10/11).

Shredder GUI has some features like online tablebase support and the possibility to open it more than once (so you can make matches on a 8 core computer where every engine uses one core), also the fischer random chess support is good (but Aquarium will support it, too).

Arena is for free, but database and analysis features are very limited. In general the other GUIs have much more features (espacially Aquarium). For engine vs. engine matches, Arena is one of the best choices.