The rating lists say that Houdini 2 is best. But before you spend any money there are a couple of free engines which are at least at par with Rybka:
Stockfish, Critter, IvanHoe, Houdini 1.5
Your favourite search engine will direct you to the download links.
Pls suggest as I plan to use it on my deep Fritz 13 GUI... deep Fritz 13 sucks!!! I am also grasteful if you suggest me the websites for the latest mp edition