The best solution for WordPress optimization we have discovered is W3 Total Cache plugin. It can be found at http://WordPress.org/extend/plugins/w3-total-cache/. However, for best results, it should be configured properly.

Here are settings that achieved best results during our tests:

1. Page Cache = Disk (Enhanced)

2. Minify Cache = Opcode

3. Database Cache = Opcode

4. Object Cache = Opcode

For Opcode you can install any of PHP caching engines like APC, XCache or eAccelerator .

However, you should understand that website speed depends on many other things, like mysql database speed, number of static files per page, different plugins, etc.