For production you generally don't need to care about which JavaScript engine is the fastest. Your page should work in all common browsers, period.

However, in certain projects where you are free to choose which browsers to support (such as hobby projects or projects for internal use), you'll find that the performance in different browsers varies a lot.

The two browsers I find to maintain top performance are Mozilla Firefox with its Spider Monkey engine and Google Chrome with its V8 engine. Apple Safari is also one of the fastest browsers with its Nitro engine, new in 4.0. They use new methods for compiling the scripts, making them perform much, much faster than before. In the future, more and more browsers will move towards this technology, as JavaScript is becoming one of the main technologies for interactive user content on the web. (I haven't mentioned Opera 10 here because I haven't personally tested it extensively, but I've heard it's faster at running JavaScript too.)