Which chess engine?

Hey all. I'd like to download a good chess engine and I'm wondering which one you all recommend/where I should dopwnload it. Thanks!

Rybka is the best engine; it's been proven.

I've downloaded over one hundred free engines for Arena, Winboard, and the Fritz GUI. More than a dozen have either not worked at all, or had some sort of bug that hindered smooth operation. I delete the bad ones and move on the others. I only need 64 engines. More would be excessive.

Here's a free Rybka download that works:

http://www.chess.com/download/view/portable-winboardx--rybka-22---updated-setup

wait for rybka 4 and buy it with aquarium

Firebird is free, open source, and stronger than Rybka.

I bit the bullet and bought Shredder 12. While it cost a few dollars, it works flawlessly for me, and is very intuitive. Feels like you are playing a real person.